Allow to add a toolchain only - without updating settings.xml, JAVA_HOME and PATH. Only toolchains.xml must be extended.

[mhoffrog]

Description:
Allow to add another toolchain only by a subsequent setup-java - only toolchains.xml must be updated with the additional JDK setup and the JDK specific JAVA_HOME_<jdk version> must be set.
No update of settings.xml, no add to PATH, no update of JAVA_HOME.

Justification:
In my scenario I run a first setup-java configuring settings.xml and setting up my JAVA_HOME JDK.
Then I determine a probable additionally required toolchain version from the compile target setting of my Maven projects POM.
If this is requiring a different JDK, then I am running another setup-java for this toolchain.
In this case I want to get the toolchains.xml being extended only and to preserve JAVA_HOME, PATH and settings.xml from my first setup-java.
Currently I am working around this requirement by a step preserving JAVA_HOME, PATH and settings.xml before the setup-java step and restoring those after that step. This is working fine so far.
Another config property for setup-java like e.g. add-toolchain-only would save those efforts!

Are you willing to submit a PR?
Yes - cannot promise a timeline yet.

[IvanZosimov]

Hi, @mhoffrog 👋 Thank you for the feature request, we will take a look and get back to your with our decision.

[TWiStErRob]

I would like this too, use case similar to https://github.com/orgs/community/discussions/73309 and https://github.com/orgs/community/discussions/51280

Pretty much anyone who provides a composite action that runs a jar or JDK commands is affected. I have no idea how others do it, probably delegate the labor to every user 😞.

[mhoffrog]

@TWiStErRob I did file PR #553 already and did keep it up to date with recent master - s. #553 (comment).
Unfortunately I do not get any further feedback nor answer on my comments on this PR - its a bit frustrating just getting no reaction for the time being.
@IvanZosimov Maybe you are all pretty busy - would there be a person in the team to assign for review or to put attention to this item?