ethclient, internal/ethapi: add support for EIP-695 (eth_chainId) (#19694)

EIP-695 was written in 2017. Parity and Infura have support for this
method and we should, too.

internal/ethapi/api.go

@@ -529,6 +529,11 @@ func NewPublicBlockChainAPI(b Backend) *PublicBlockChainAPI {
 	return &PublicBlockChainAPI{b}
 }
 
+// ChainId returns the chainID value for transaction replay protection.
+func (s *PublicBlockChainAPI) ChainId() *hexutil.Big {
+	return (*hexutil.Big)(s.b.ChainConfig().ChainID)
+}
+
 // BlockNumber returns the block number of the chain head.
 func (s *PublicBlockChainAPI) BlockNumber() hexutil.Uint64 {
 	header, _ := s.b.HeaderByNumber(context.Background(), rpc.LatestBlockNumber) // latest header should always be available