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.

ethclient/ethclient.go

@@ -61,6 +61,16 @@ func (ec *Client) Close() {
 
 // Blockchain Access
 
+// ChainId retrieves the current chain ID for transaction replay protection.
+func (ec *Client) ChainID(ctx context.Context) (*big.Int, error) {
+	var result hexutil.Big
+	err := ec.c.CallContext(ctx, &result, "eth_chainId")
+	if err != nil {
+		return nil, err
+	}
+	return (*big.Int)(&result), err
+}
+
 // BlockByHash returns the given full block.
 //
 // Note that loading full blocks requires two requests. Use HeaderByHash