Support externally managed contract executables (CAP-85) #1559

What

Handle the new SCVal and contract-executable variants from CAP-85:

• SCV_EXECUTABLE_TAG added to SCValType (and SCLedgerKeyType).
• CONTRACT_EXECUTABLE_EXTERNAL_REF added to ContractExecutableType, with the ContractExecutableExternalRef struct (owner address + tag string).

Work items:

• src/base/scval.ts — add a case for SCV_EXECUTABLE_TAG in scValToNative and nativeToScVal. If no native mapping fits, add an explicit unsupported fallthrough so it fails loudly rather than misbehaving.
• Review src/contract and src/rpc for deploy helpers that may want a convenience path for creating or updating contracts from an external executable ref.
• Tests for the new SCVal conversion.

Why

CAP-85 lets one contract reference another's executable, enabling atomic upgrades across a fleet of contracts. Clients that read ledger entries or contract data will encounter the new SCVal and executable variants.

Notes

• Status: Accepted.
• Most of this flows in with the XDR regen; the work here is the hand-written SCVal helpers and any deploy convenience.
• The new host functions (create_external_ref_contract, etc.) are contract-side (Rust). Not this SDK's concern.
• Ref: https://github.com/stellar/stellar-protocol/blob/master/core/cap-0085.md