gRPC client doesn't work with Service Discovery scheme resolution syntax

The format used to support URI scheme resolution recently added to service discovery (e.g. `https+http://`) does not work with the gRPC client stack. The [`GrpcChannel` class](https://github.com/grpc/grpc-dotnet/blob/933023b6f818ec54dfe84e7d31ec50dfe5044c13/src/Grpc.Net.Client/GrpcChannel.cs#L105) does a lot of initialization in its constructor based on the scheme of the address and special-cases `https://` and `http://` for numerous operations including `ResolverFactory` resolution and credential selection.

@JamesNK interested in your thoughts on how we might go about fixing this. I messed about a bit with a custom `ResolverFactory` and got further but seems we'd have to integrate more deeply to make the syntax work or update `GrpcChannel` to tolerate it some other way.