ethclient: add NetworkID method (#14791)

There is currently no simple way to obtain the network ID from a Client. 
This adds a NetworkID method that wraps the net_version JSON-RPC call.

ethclient/ethclient.go

@@ -256,6 +256,19 @@ func (ec *Client) SubscribeNewHead(ctx context.Context, ch chan<- *types.Header)
 
 // State Access
 
+// NetworkID returns the network ID (also known as the chain ID) for this chain.
+func (ec *Client) NetworkID(ctx context.Context) (*big.Int, error) {
+	version := new(big.Int)
+	var ver string
+	if err := ec.c.CallContext(ctx, &ver, "net_version"); err != nil {
+		return nil, err
+	}
+	if _, ok := version.SetString(ver, 10); !ok {
+		return nil, fmt.Errorf("invalid net_version result %q", ver)
+	}
+	return version, nil
+}
+
 // BalanceAt returns the wei balance of the given account.
 // The block number can be nil, in which case the balance is taken from the latest known block.
 func (ec *Client) BalanceAt(ctx context.Context, account common.Address, blockNumber *big.Int) (*big.Int, error) {