ethclient: add RevertErrorData function and example (#30669)

Here I'm adding a new helper function that extracts the revert reason of
a contract call. Unfortunately, this aspect of the API is underspecified.
See these spec issues for more detail:

- ethereum/execution-apis#232
- ethereum/execution-apis#463
- ethereum/execution-apis#523

The function added here only works with Geth-like servers that return
error code `3`. We will not be able to support all possible servers.
However, if there is a specific server implementation that makes it
possible to extract the same info, we could add it in the same function
as well.

---------

Co-authored-by: Marius van der Wijden <[email protected]>

ethclient/ethclient.go

@@ -630,6 +630,23 @@ func (ec *Client) SendTransaction(ctx context.Context, tx *types.Transaction) er
 	return ec.c.CallContext(ctx, nil, "eth_sendRawTransaction", hexutil.Encode(data))
 }
 
+// RevertErrorData returns the 'revert reason' data of a contract call.
+//
+// This can be used with CallContract and EstimateGas, and only when the server is Geth.
+func RevertErrorData(err error) ([]byte, bool) {
+	var ec rpc.Error
+	var ed rpc.DataError
+	if errors.As(err, &ec) && errors.As(err, &ed) && ec.ErrorCode() == 3 {
+		if eds, ok := ed.ErrorData().(string); ok {
+			revertData, err := hexutil.Decode(eds)
+			if err == nil {
+				return revertData, true
+			}
+		}
+	}
+	return nil, false
+}
+
 func toBlockNumArg(number *big.Int) string {
 	if number == nil {
 		return "latest"