gRPC directives for pekko-http

[timw]

This adds pekko-http directives that detect grpc/grpc-web requests.

This support integration of grpc endpoints alongside other HTTP requests on the same server binding (e.g. HTTP based /healthz handlers etc.) and different handling for different protocol types (e.g. enforcing Strict-Transport-Security for grpc-web requests).

Java and Scala DSLs are provided.
Currently the Java DSL uses the @InternalApi RouteHandler from pekk-http - this could be copied in-tree if that's not considered maintainable.

The GrpcProtocol.detect hasn't been used, as it doesn't check the main type of the MediaType at present - alternatively that could be changed and used for this purpose as well.

[timw]

Looks like some scalafmt problems - something up with the IntelliJ scalafmt execution perhaps.

[pjfanning]

@timw could you add javadoc with @since 2.0.0 to the new public methods.
Is there a way some test coverage could be added?

[timw]

@timw could you add javadoc with @since 2.0.0 to the new public methods.

Done.

Is there a way some test coverage could be added?

There's a spec for the scaladsl directives there already - do we want coverage of the javadsl as well?

[pjfanning]

Can you move keep the pekko imports together?

[raboof]

It's not super obvious what use cases this should cover. We already have some examples of Pekko HTTP integrations documented at https://pekko.apache.org/docs/pekko-grpc/current/server/pekko-http.html - perhaps it would be good to add some examples of typical use cases for this API to that page?