4.x: Make HttpRouting an interface

[VerKWer]

Description

Currently HttpRoute is an interface whereas HttpRouting isn't. Being able to implement your own HttpRouting type can be useful when moving an existing service to Helidon; for example if there already is custom routing logic for it. In addition, making this change seems ideologically correct because it allows for more configurability, and almost everything else that can be specified in a WebServer builder is an interface.

Another issue that's addressed as a side-effect is that while other routing classes (e.g. GrpcRouting) aren't final, using a custom implementation isn't easily possible. The reason for this is that the RouterImpl indexes Routing instances by their class and its routing method is always called with a fixed class. Therefore, when trying to use a custom implementation, it wouldn't be found.

Documentation

The HttpRouting class is replaced by an interface and all implementation/builder code previously in it is moved to the new package-private HttpRoutingImpl class. This should not break existing applications because HttpRouting was final (so no subclasses can be defined) and its constructor was private (so it cannot be called directly).

The routingType() method is added to the Routing interface with a default implementation consistent with the previous behaviour. Its result is used by RouterImpl to determine the correct Routing instance to use. I'm not sure returning a Class here is the best solution, but it is what was previously used.

Resolves #8572

[tomas-langer]

Hello @VerKWer - I have approved the pipeline run.
In general we start from an issue in our issue tracker. If there is none yet created, kindly create it and link it to your PR. If there is one, just link it. This is the kind of work that requires approval of design. Although I agree with the approach you have taken, it would be better to discuss this first on an issue, or through our Slack channel, to make sure we are OK with such a change.

Also this is going into the internals of WebServer, so I will need to spend a bit of time making sure this does not introduce any kind of backward incompatibility.

Thanks for your contribution!

[VerKWer]

Hey @tomas-langer! Thank you for having a look at the PR and pointing out the proper procedures; this is my first time contributing to Helidon, and I'll gladly create an issue to link to.

I did have similar thoughts about maybe first reaching out to check whether my contribution was welcome. To that end, I asked in the Nima Slack channel at the time but realise only now that that was more than four months ago on November 14 (time really does fly). I can't see the thread anymore, but I think it was you, who answered back then that you'd be open for the change, which was very encouraging. Still, I'll make sure to communicate better for future contributions, and to always create an issue first.