Use GH CLI, check for existing trailers and clean up code

Use GH CLI in place of manual CURL calls, add check for existing
trailers and clean up code logic:
- Use GH CLI to fetch and parse PR information
- Use `action/checkout` to checkout the PR branch and add `token` input
  to properly handle checkout access permissions
- Add step to check if trailers exist and avoid re-adding existing
  trailers. Add inputs to control if the action will only check for the
  existence of required trailers and, optionally, fail if the trailers
  do not exist. Add output with the check result, updated by the
  addition of the trailers.
- Clean up steps, variable names and use `GITHUB_OUTPUT` instead of
  `GITHUB_ENV`

Signed-off-by: Kostis Papazafeiropoulos <papazof@gmail.com>

Author: papazof

README.md

@@ -1,14 +1,16 @@
-This action parses the PR branch and PR info and adds git trailers
-to persist PR metadata history in the commit messages.
+# git-trailers
 
-Example usage:
+This action parses information for a pull request and adds git trailers to
+persist PR metadata history in the commit messages.
 
-Create a workflow file for an action to be triggered on PR approval.
+## Example usage
+
+Create a workflow to run the action on PR approval.
 
 eg:
-`.github/workflows/pr-approve.yml`:
+`.github/workflows/pr-approval.yml`:
 
-```
+```yaml
 name: Add Git trailers for PRs
 
 on:
@@ -17,24 +19,15 @@ on:
 
 jobs:
   git-trailers:
-    runs-on: [self-hosted]
     if: github.event.review.state == 'approved'
-
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
     steps:
-      - name: Cleanup previous jobs
-        run: |
-          echo "Cleaning up previous runs"
-          sudo rm -rf ${{ github.workspace }}/*
-          sudo rm -rf ${{ github.workspace }}/.??*
-
-      - name: Checkout Repository
-        uses: actions/checkout@v2
-        with:
-          fetch-depth: 0
-
-      - name: Do git-trailers
-        uses: nubificus/git-trailers@v1
+      - name: Add git trailers
+        uses: nubificus/git-trailers@feat_gh_checkout_rebase
 ```
 
-Upon approval, the action would overwrite current commits against `main` (or `master`)
-adding trailers for reviewers, approvers and the PR#.
+Upon approval, the action will edit the PR commit messages adding trailers for
+the PR number, reviewers and approvers and update the PR (by force-pushing the
+final branch).