Introduce nightly/manual Early Access JDK build

[famod]

Resolves #15575 (the main part)

This job includes virtually everything from CI, except:

• resteasy-reactive-testsuite, but since I'm planning on introducing a new tcks submodule for this soon (to resolve task 3 in Incremental build improvements #15686), it will be executed automatically by then
• native tests (which are based on GraalVM anyway)

The Gradle ITs are also executed via Maven (like you would do locally today). CI will (hopefully) switch to the same mode soon via #15698.

The job takes over 4 hours (in my fork).

Once in main (also works for forks), you can trigger this workflow manually from whichever branch and for whichever (Zulu) JDK version you like:

There are quite a few problems with JDK 16 as can be seen in my fork: https://github.com/famod/quarkus/actions/runs/651661445
In contrast, all is green with 11: https://github.com/famod/quarkus/actions/runs/651648963

PS: This comes with a nice test report as shown here: https://quarkusio.zulipchat.com/#narrow/stream/187038-dev/topic/CI.3A.20nicer.20display.20of.20test.20failures

[famod]

CI results are unrelated. This new file does not have any influence on anything that CI is doing.

[famod]

/cc @gsmet in case you missed this.

Btw, JDK 16 reaches GA tomorrow already! 😮

[gemmellr]

Came to see if anyone had reported the JDK16 build failure yet now that it has been released (updated some CI jobs elsewhere and hit it), seems like this PR/issue covers it somewhat.

Presumably once those are fixed the main CI job would switch over to 16 and this type of job would target JDK17EA which has been available for a while now too.

[famod]

@gemmellr

Presumably once those are fixed the main CI job would switch over to 16 and this type of job would target JDK17EA which has been available for a while now too.

Yes, that's the (or at least my) plan.

@gsmet WDYT? I think this is good to merge. We should make some progress on the JDK16 front.