For remote RequestSource, allow alternative option to RequestSpecifiers for full requests

[dubious90]

For more complete control, have an alternative option for RequestSource where the remote source specifies the whole requests, either in an existing protobuf definition or as serialized json/yaml.

This should not replace the existing RequestSpecifier system, which provides separate value. Maybe this could exist as a separate rpc stream? Or the proto contract could specify allowing one or the other