Support 'details' from the RPC Error Model

[kriswuollett]

It currently isn't possible to fully take advantage of and interact with (and produce) gRPC services that make use of the full RPC Error Model because the GrpcError class is missing the details field as defined in google/rpc/status.proto.

One example use case out of many is being able to access retry_delay for behaving nicely with a service.

This would help developers avoid baking in custom error details into their custom response message definitions themselves. My current workaround for Dart (and the other languages that don't support this yet) is to encode google.rpc.Status into (a JSON String field in) the message field.

[hrpath]

Any news on this front, this could help a lot adoption or cut it down.
Thx

[acoutts]

@kriswuollett I got started on this, please see my PR here: #349