refactor(anvil-core): make ImpersonatedTransaction and BlockInfo generic

[stevencartavia]

Motivation

Preparation for phasing out FoundryNetwork by making anvil-core RPC types generic over Network. Currently MaybeImpersonatedTransaction and BlockInfo are hardcoded to FoundryTxEnvelope and FoundryReceiptEnvelope, preventing reuse with different network implementations.

Solution

1. Remove into_rpc_transaction from anvil-core

• Deleted into_rpc_transaction() and From<MaybeImpersonatedTransaction> for RpcTransaction from anvil-core
• Inlined the conversion logic into transaction_build() in anvil/src/eth/backend/mem/mod.rs where it belongs (anvil-level concern, not a core type concern)

2. Extract ImpersonatedTxEnvelope trait

• Defines tx_hash(), recover_signer(), impersonated_hash() — the interface MaybeImpersonatedTransaction needs from its inner envelope
• Implemented for FoundryTxEnvelope, decoupling anvil-core from the concrete type

3. Make ImpersonatedTransaction<T> generic

• Renamed struct to ImpersonatedTransaction<T> with all trait impls (Typed2718, Encodable2718, Encodable, Decodable, AsRef, Deref) generic over T
• Type alias MaybeImpersonatedTransaction = ImpersonatedTransaction<FoundryTxEnvelope> preserves backward compat — zero changes needed in downstream code

4. Make TypedBlockInfo<T, R> generic

• Renamed struct to TypedBlockInfo<T, R>, generic over transaction and receipt types
• Type alias BlockInfo = TypedBlockInfo<MaybeImpersonatedTransaction, FoundryReceiptEnvelope> preserves backward compat
• Added create_typed_block<T> generic helper alongside existing create_block

PR Checklist

• Added Tests
• Added Documentation
• Breaking changes