Introduce NameResolver.Args.Extensions

[jdcormie]

grpc-binder's upcoming IntentNameResolver needs to know the target Android user so it can resolve target URIs in the correct place. Unfortunately, Android's built in intent:// URI scheme has no way to specify a user and in fact the android.os.UserHandle object can't reasonably be encoded as a String at all.

We solve this problem by extending NameResolver.Args to permit externally defined arguments using the same type-safe and domain-specific Key<T> pattern used by CallOptions, Context and CreateSubchannelArgs. New "extension" arguments could apply to all NameResolvers of a certain URI scheme, to all NameResolvers producing a particular type of java.net.SocketAddress, or even to a specific NameResolver subclass.

A follow up PR will move the target UserHandle from being a property of the ManagedChannel to being a property of the SocketAddress.