feat: Per request instance factory for Java DSL

[johanandren]

Functionality for integrating Akka gRPC generated Java services as endpoints in Akka SDK.

[octonato]

LGTM, with a question...

Did you call it ScalaHandler because it's based on a Scala PF?

It's a little weird that a parameter called generate_scala_handler_factory generates a Java file.

[johanandren]

It's ScalaHandler because the public API of it is Scala (PF and Future) for folding into a Scala HTTP server, compared to the normal handler factory which has API to create akka.japi.function.Function and CompletionStage to be bound using the Java API of Akka HTTP

[johanandren]

And, should I perhaps add, generated as Java to not require Akka SDK projects to have a Scala compilation step.