feat: add pull request cache behavior

[neilime]

When pull_request_cache input is set, this action instructs docker/build-push-action to import and export cache of the pull request.

cache-from: |
  type=registry,ref=IMAGE:pr-1
  type=registry,ref=IMAGE:main
cache-to: type=registry,ref=IMAGE:pr-1,mode=max

This is useful when you want to improve build cache between consecutive commits for a same pull request.

[neilime]

@int128 can you have a check on this please?

[artificial-aidan]

Just tested this and it is working great for us.

[int128]

Thank you for the great contribution!
I'd like to add a commit of some suggestions.