Add the validate.yml PR workflow for changed ckans

[Maximilian-Nesslauer]

Goal

Validate hand-edited .ckan files in PRs the same way KSP-CKAN/KSP2-CKAN-meta does with its .github/workflows/validate.yml, completing the "populate the .github folders" part of the "make repos" step of KSP-CKAN/CKAN#4457.

Tasks

• Add .github/workflows/validate.yml mirroring KSP2-CKAN-meta's: on PRs and pushes to main, run the xKAN-meta_testing action with game: KSA and source: commits against the changed files, with the download cache step.

Gate / sequencing

Same gate as the KSA-NetKAN workflows: the action runs the upstream-built kspckan/metadata image, which only accepts --game KSA after the client lands in KSP-CKAN/CKAN.
Merge the workflow whenever; it starts passing after the upstream deploy.

Part of KSP-CKAN/CKAN#4457.