feat(rpc): impl getMultipleAccounts RPC

[prestonsn]

Implements getMultipleAccounts on accounts db two, building on top of the getAccountInfo impl.

getMultipleAccounts: Sig vs Agave Testnet (v3.1.8)

Test Case	Result
commitment: processed	Match -- same slot, same accounts
commitment: confirmed	Match -- same slot, same accounts
commitment: finalized	Match -- same slot, same accounts
base64 encoding	Match -- identical data and format
base58 encoding	Match -- identical data and format
base64+zstd encoding	Match -- identical compressed data
dataSlice (offset=0, length=8)	Match -- correctly sliced data
No config (defaults to base64)	Match -- same default behavior
Empty pubkeys array	Match -- both return []
Single pubkey	Match -- returns single-element array
Non-existent account in array	Match -- both return null in array
No params	Match -- both return error -32602

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/rpc/methods.zig	81.13% <ø> (ø)
src/rpc/test_serialize.zig	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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