add workflow to post comment comparing updated deps

[matt-codecov]

this is a reusable workflow that will automatically post/update a PR comment when shared is updated. the comment contains a link to a GitHub page that will show all of the commits/changes between the old shared version and the new shared version

here's a screenshot that shows a comment that was edited in place when the PR was updated

it takes the dep as an input, so we could actually use this for the test results parser or other git-based pip dependencies. if somebody wants to teach it javascript, we could use it for gazebo too

you use it like this:

jobs:
  post-dep-comment:
    name: Post dep comment
    uses: codecov/gha-workflows/.github/workflows/diff-dep.yml@pr30
    with:
      dep: 'shared'

test repo / PR where i tested this: https://github.com/codecov/test-workflow/pull/2

• Workflow run when no comment already exists
• Workflow run when a comment already exists

[matt-codecov]

i don't know what happened to this PR. it has 0 commits and i can't reopen it. moved to #31