Add support for gas key transactions, which require handling multiple
nonces per key.
I completely leave out the fund accounting and balance accounting
changes from this PR and we can add them later to keep scope small. I
added a TODO for this, and for now account's balance continues to pay
all `tx_cost`.
(This is a "safe" approach, gas keys are just like ordinary access keys
with multiple nonces with this change).

* Updated transaction verification logic to distinguish between regular
and gas key transactions, calling
`verify_and_charge_gas_key_tx_ephemeral` for the latter and updating
nonce values accordingly.
* Modified the runtime to prefetch gas key nonces in parallel with
accounts and access keys, using a new `gas_key_nonces`. The cache in
`prepare_transactions_extra` is also modified to accommodate gas key
nonces.
* Added a new `InvalidTxError::InvalidNonceIndex` variant to handle
cases where a transaction specifies an invalid nonce index.

I took an approach of having a separate code path for gas keys based on
feedback from last attempt at this and further changes that will be
introduced for balance handling. In case we want a _unified_ code path
instead, an unpolished draft is available in
8bf4f28.

Tests in separate PR to manage diff size:
#14925

The separate code path introduces duplication, but we can mitigate that
by refactoring common parts which is done here:
#14923 (This PR is based on that
branch)


As usual I defer openapi & protocol schema changes to last step before
merging.

Add support for gas key transactions, which require handling multiple
nonces per key.
I completely leave out the fund accounting and balance accounting
changes from this PR and we can add them later to keep scope small. I
added a TODO for this, and for now account's balance continues to pay
all `tx_cost`.
(This is a "safe" approach, gas keys are just like ordinary access keys
with multiple nonces with this change).

* Updated transaction verification logic to distinguish between regular
and gas key transactions, calling
`verify_and_charge_gas_key_tx_ephemeral` for the latter and updating
nonce values accordingly.
* Modified the runtime to prefetch gas key nonces in parallel with
accounts and access keys, using a new `gas_key_nonces`. The cache in
`prepare_transactions_extra` is also modified to accommodate gas key
nonces.
* Added a new `InvalidTxError::InvalidNonceIndex` variant to handle
cases where a transaction specifies an invalid nonce index.

I took an approach of having a separate code path for gas keys based on
feedback from last attempt at this and further changes that will be
introduced for balance handling. In case we want a _unified_ code path
instead, an unpolished draft is available in
near@8bf4f28.

Tests in separate PR to manage diff size:
near#14925

The separate code path introduces duplication, but we can mitigate that
by refactoring common parts which is done here:
near#14923 (This PR is based on that
branch)


As usual I defer openapi & protocol schema changes to last step before
merging.

Adds unit testing and a test loop test for GasKeys transaction/nonce
handling.

Initially I included an "integration test" in
`chain/chain/src/runtime/tests.rs` but removed it in
near@955a176
and added a basic test loop test instead.
The reason is I don't want to expand `TestEnv` in any way and there is a
bit of a broken relation between Chain & TestEnv when it comes to the
validity period check.

In any case I am happy to bring that test back or try some suggested
workarounds or alternative testing strategies.

# Gas Keys ([NEP-611](near/NEPs#611))
Gas keys are a new type of access key that carries a pre-funded NEAR
balance and multiple independent nonce sequences. When a transaction is
signed with a gas key, gas costs are deducted from the gas key's balance
rather than the signer's account balance (deposits like NEAR transfers
still come from the account).

## Why do we need gas keys?
- Enabling SPICE without a DoS vector. The SPICE project (Separation of
Execution from Consensus) decouples transaction execution from block
production: transactions are included in blocks first, and executed
asynchronously later. This means chunk producers can no longer verify
account balances at inclusion time. An attacker could drain an account's
balance in one transaction, then flood the chain with many transactions
signed by that account. All would be accepted (since execution hasn't
caught up), consuming block space for free when they inevitably fail.
Gas keys close this gap by requiring gas to be pre-committed in the key
itself, so chunk producers can verify gas availability without access to
the latest state.

- Parallel transaction submission. Today, programmatic users who submit
many transactions in parallel must manage multiple access keys (each
with its own nonce sequence). A gas key supports up to 1,024 independent
nonce slots via a single key, removing that complexity.

# Context

- NEP (if exists): near/NEPs#611
- Implementation: 
 - near#14868
 - near#14869
 - near#14883
 - near#14891
 - near#14892
 - near#14911
 - near#14924
 - near#14925
 - near#14930
 - near#14933
 - near#14969
 - near#14985
 - near#15033
 - near#15034
 - near#15067
 - near#15068
 - near#15100
 - near#15101
 - near#15186

# Testing and QA
- unit tests
- test-loop:
https://github.com/near/nearcore/blob/master/test-loop-tests/src/tests/gas_keys.rs
- forknet (automated): near#15207
- forknet (manual testing, planned)

# Checklist
- [x] Link to nightly nayduck run:
https://nayduck.nearone.org/#/run/4756
- [x] Update CHANGELOG.md to include this protocol feature in the
`Unreleased` section.