rpc: return `chainId` for legacy txs using EIP-155 signature + don't accept `chainid` in `TransactionArgs`

[lightclient]

This issue came up in a discussion here: ethereum/execution-apis#239

I think the correct approach is to set chainId for legacy txs that take advantage of EIP-155. If we want to take it a step farther, we could say that chainId = 0 refers to a tx valid on any chain. This is essentially what the tx.ChainId() method does.

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Additionally, accepting chainId in TransactionArgs seems superfluous. If chainId doesn't match with the client it should probably be an error anyways. Seems simpler to just remove it altogether.

[lightclient]

So I'll make sure EIP-155 legacy txs also return chainId over RPC and that if chainId in tx args does not match the local chain id we will always error.