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_test.go

@@ -319,3 +319,19 @@ func TestTransactionInBlockInterrupted(t *testing.T) {
 		t.Fatal("error should not be nil")
 	}
 }
+
+func TestChainID(t *testing.T) {
+	backend, _ := newTestBackend(t)
+	client, _ := backend.Attach()
+	defer backend.Stop()
+	defer client.Close()
+	ec := NewClient(client)
+
+	id, err := ec.ChainID(context.Background())
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if id == nil || id.Cmp(params.AllEthashProtocolChanges.ChainID) != 0 {
+		t.Fatalf("ChainID returned wrong number: %+v", id)
+	}
+}