Proxy for a io.grpc.ManagedChannelBuilder with unknown type argument type

Hides the wildcard type argument to ManagedChannelBuilder so that type inference will not infer the more correct argument A forSome { type A <: ManagedChannelBuilder[A] }, which is an existential type that cannot be expressed by wildcards. This works only because Scala does not check the bounds of wildcard type arguments.

See https://stackoverflow.com/a/5520212 for some background on existential types