add automation action for plugin_cache.json

[jmartin-tech]

Maintain the distributed plugin_cache.json via github action preformed on merge of PRs to main.

• more tolerant cache validation
  • cache rebuild allows for and empty cache file containing only {}
• new automation action triggered on push to main or manual branch trigger
  • run on push to main
  • only commit when changes are found
  • cancel if a new commit is pushed to same branch including main ( avoids race conditions on cache update )
  • do not commit changes on branch trigger ( only log if changes exist )
  • do not execute when only cache file is updated ( avoids commit loop by automation )

Noted quirks:

• concurrency limit - workflow cancelled by a new commit will mark the commit as failing automation, the subsequent completion will mark only the last push as passing tests
• Commit by automation to main does not trigger a full new test run ( metadata only change makes this reasonable )