Add support for RPC status

[SebastianThiebaud]

This closes #258

The new changes provide a way for consumer of grpc-swift to get access to the data stored in the grpc-status-details-bin of metadata.

Here's how to use it in the consumer app:

// Register message types that could be returned in the details field of Status
Google_Protobuf_Any.register(messageType: CustomError.self)

_ = try service.doSomething(request) { response, result in
  result.trailingMetadata?.status?.details // will contain a list of messages that carry the error details
}

[MrMage]

Thank you for this change! I am a bit concerned about adding a compiled .pb.swift file to the library source, given that that would require us to also vendor the source .proto file (and add a means of updating that file whenever it or the SwiftProtobuf plugin change).

Also, would it be possible to add tests for the added code as well?

[SebastianThiebaud]

@MrMage I added a simple test.

[MrMage]

Great, thank you for the changes! Looks good to me, will merge once the data accessor is moved, if you don't mind.