Epic: Install multiple JDK distributions in one step

setup-java already documents installing multiple versions of a single distribution (install-multiple-jdks). Several requests ask to install multiple distributions (e.g. Temurin and Zulu, or different distros per version) in a single step. This epic tracks that capability and the related approaches.

Sub-requests / related

• Allow to install multiple JDK distributions #945 — Allow installing multiple JDK distributions (general request) — closed as not planned; achievable today via strategy.matrix (cross-vendor testing) or stacked setup-java steps that accumulate in toolchains.xml (multi-distro in one job). See comment.
• Add support for multiple distributions via the Disco API #142 — Add support for multiple distributions via the Foojay Disco API (one implementation approach: a generic resolver so any Disco-supported distro works)

Relationship to other work

• Complements the set-default primitive (PR feat: Add set-default option #1017) and the "toolchains-only" mode (Add a "toolchains-only" install mode (layered on the set-default primitive) #1047): installing several distributions raises the question of which becomes the default / lands in JAVA_HOME vs is only registered as a Maven toolchain. Design should compose with those.
• Overlaps conceptually with the per-version distribution matrix users do today via strategy.matrix.

Design questions

• Input shape: parallel arrays (distribution + java-version lists) vs a structured/multiline mapping.
• Default selection: which installed JDK populates JAVA_HOME / PATH (ties into set-default).
• Whether to adopt a generic Disco/Foojay resolver (Add support for multiple distributions via the Disco API #142) vs extend the existing per-distribution installers.

Notes

Coordination/visibility epic. #945 was closed as not planned (existing matrix + stacked-steps patterns cover the use cases). A usage question about the existing multi-JDK feature (#586) was answered and closed separately; it is not part of this feature work.