InstantiatingGrpcChannelProvider - Support non-host:port target strings

[hbchai]

Currently, InstantiatingGrpcChannelProvider endpoints must be host:port strings. This is a request to support target strings, e.g. that can be resolved using custom name resolvers. Implementation-wise, it seems that allowing a target string to be passed through to ManagedChannelBuilder.forTarget() instead of forAddress() would be sufficient.

Primary use cases are for testing.