feat: Add set-default option

[gsmet]

Description:

Add a set-default option allowing to install an additional JDK without making it the default one.

I have wanted this for quite a long time as I'm running custom GitHub Actions with Java, which might require a specific JDK and I don't want to pollute the JDK that is used by the overall workflow calling the action.
And I'm apparently not alone as there was a preexisting issue.

Related issue:

Fixes #560

Check list:

• Mark if documentation changes are required.
• Mark if tests were added or updated to cover the changes.

[brunoborges]

Cross-linking for coordination: this set-default option overlaps conceptually with the older PR #553 (issue #552), which also lets you install a JDK without mutating JAVA_HOME/PATH.

set-default: false here is effectively a cleaner, single-flag expression of #553's update-env-javahome: false + update-env-path: false (both also deliberately preserve JAVA_HOME_<major>_<arch>). We're treating set-default as the canonical primitive for "don't make this JDK the default."

The one piece #553 covered that this PR does not is a Maven toolchains-only install (register the JDK in toolchains.xml only). That remaining gap has been split out into #1047, scoped to build on top of set-default rather than re-implement env skipping. PR #553 is being closed in favor of this PR + #1047.

No change requested here — just establishing the lineage: #560 (this PR) and #552 → #1047.