Conditional extension dependencies support for Maven-based projects

[aloubyansky]

This is PR enables conditional extension dependencies for Maven-based projects. The doc describing how they work and how to configured them is included.
The gradle support will come in a different PR.

Also this PR adds quarkus:dependency-tree Maven goal to the quarkus-maven-plugin (a copy of quarkus-bootstrap-maven-plugin:build-tree which is now deprecated)

[aloubyansky]

@FroMage mentioned to me I needed a better example in the doc. I'll improve that. We'll need to document the Gradle part as well when it's ready, so the doc will be evolving. I just wanted to get the code changes in ASAP to see whether they introduce any regressions.

[dmlloyd]

IMO you should use SmallRye Common's version facility. For no particular reason 😁

[maxandersen]

I see from zulip conversations that gradle is not as straight forward - so are we suggesting to put this in but not enable it yet until gradle appraoch figured out or whats the intended approach ?

[aloubyansky]

Yes

[kenfinnigan]

It looks like it would solve my use case nicely for OpenTelemetry and Vert.x optional dependencies.

One comment is whether the conditional dependencies doc should be linked from the extension writing doc, otherwise, I'm not sure how extension writers would know to search for this?

[aloubyansky]

The doc will definitely need to be made easy to locate. It's just it's not yet in its final state (the feature isn't yet fully supported, i.e. the gradle side of it) and linking it from the published ones is currently to early.

[aloubyansky]

I created the main enhancement issue to track the rest of the tasks #15844