Check cache for key, skipping download and extraction.

Describe the enhancement
Quickly check if the cache exists without having to download and extract a potentially large cache.

Code Snippet

restoreCache(paths, key, [restoreKey], {
    checkKeyOnly: true
 })

checkKeyOnly?: boolean was added to the DownloadOptions interface.

Additional information
I have written a reference implementation (could be used as a PR) for this feature which can be found at:
https://github.com/CareerJSM/toolkit/tree/feature/check-cache-key-only.

In addition, I have updated the action/cache repository to use this new feature in the toolkit:
https://github.com/CareerJSM/cache/tree/feature/check-cache-key-only

I will submit the PR to the actions/cache repository once the toolkit supports this capability.

Edit - 2022-12-19

Since this doesn't appear to be going anywhere, and my branches were falling behind, I re-approached this issue and am now using the gh-actions-cache extension to check the cache.

E.g.

  - name: Check if cache key exists
    id: check-if-cache-current
    env:
      GH_TOKEN: ${{ github.token }}
    run: |
      gh extension install actions/gh-actions-cache
      KEY=${{ steps.gem-cache-keys.outputs.full-key }}
      CACHE_HIT=$(gh actions-cache list --key $KEY | grep -q $KEY && echo 'true' || echo 'false')
      echo "cache-hit=$CACHE_HIT" >> $GITHUB_OUTPUT
  - name: Restore Gem Cache
    if: ${{ steps.check-if-cache-current.outputs.cache-hit != 'true' }}
    id: restore-gem-cache
    uses: actions/cache@v3
    with:
      path: ...

Start with the DownloadOptions interface and the restoreCache entry point described in the issue, then compare the reference implementation in the CareerJSM/toolkit check-cache-key-only branch. Verify how the action/cache branch expects the option to behave. Done means a cache key can be checked without downloading or extracting the cache, with the related behavior covered by the project’s tests.