In [NEP-611](https://github.com/near/NEPs/pull/611/files), it is
specified:
> * Cost of `TransferToGasKey` and `WithdrawFromGasKey` will be based on
`Transfer`, as it is the most similar existing action.
`WithdrawFromGasKey` and `TransferToGasKey` do additional trie
modifications on sending and on execution. These will be accounted for
based on trie operations.

This PR provides the concrete implementation for this specification.


  **Send Fees**
  _TransferToGasKey / WithdrawFromGasKey_:
  gas_key_transfer_base.send + gas_key_key_byte.send * public_key_len

  _AddKey (gas key variant) - permission_send_fees_:
add_{function_call_key_base,full_access_key}.send +
...method_name_bytes... + gas_key_byte.send * GasKeyInfo::borsh_len()

  **Exec Fees**
  _TransferToGasKey / WithdrawFromGasKey_:
gas_key_transfer_base.exec + gas_key_byte.exec * ak_key_len +
gas_key_byte.exec * estimated_value_len
where ak_key_len = access_key_key_len(receiver_id, public_key) and
estimated_value_len = AccessKey::min_gas_key_borsh_len().

  _AddKey (gas key variant) - permission_exec_fees_:
add_{function_call_key_base,full_access_key}.exec +
...method_name_bytes... + num_nonces * (gas_key_nonce_write_base.exec +
gas_key_byte.exec * nonce_key_len + gas_key_byte.exec * nonce_value_len)
where nonce_key_len = gas_key_nonce_key_len(account_id, public_key) and
nonce_value_len = NONCE_VALUE_LEN.

  **Values (derived from ExtCosts)**
gas_key_transfer_base: send=transfer;
exec=send+storage_read_base+storage_write_base
  ────────────────────────────────────────
gas_key_byte: send=new_data_receipt_byte;
exec=storage_read_key_byte+storage_write_key_byte
  ────────────────────────────────────────
  gas_key_nonce_write_base: send=0; exec=storage_write_base

N = `num_nonces`

- DeleteKey & DeleteAccount can only charge variable compute for num
nonces, will address in a separate
[PR](#15068).
- Adds `AccessKey::NONCE_VALUE_LEN`, `min_gas_key_borsh_len()`, and trie
key length helpers as shared primitives
- Uses `min_gas_key_borsh_len()` as estimated value length where the
actual gas key variant is unknown at fee computation time

# 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.