Make ProtosRegistry a class instead of singleton

[zenyui]

Is your feature request related to a problem? Please describe.
ProtoRegistry is a little hard to test/deal with as a singleton. We tried making it a class, but the ApiSerializer fails with the error:

Can't generate a Lagom client for io.superflat.lagompb.data.TestService since the following abstract methods don't return service calls or topics: protosRegistry

Describe the solution you'd like
somehow convert ProtoRegistry to a class with a companion to instantiate with reflection (easier to test, mock, pass around), but in a way that does not require doing a new reflection of proto classes for each serialized response.

Describe alternatives you've considered
leaving it as it is...

Additional context
n/a

[Tochemey]

This will only be possible when lagom service definition allow such way of injecting field via DI.