chore: create combined PR using GitHub app (#803)

* Create combined PR using GitHub app

* Fix token attribute

* Fix API for creating pull request

Reference: https://octokit.github.io/rest.js/v18#pulls-create

* Grant token to push branch

* Revert "Grant token to push branch"

* Create branch via API

* Fix syntax

* Remove creation of ref

Signed-off-by: Aman Sharma <mannu.poski10@gmail.com>

* Prepend head with username

Signed-off-by: Aman Sharma <mannu.poski10@gmail.com>

* Revert "Prepend head with username"

This reverts commit 969f877.

* Revert "Remove creation of ref"

This reverts commit 89995b3.

* Fix sha to be the revision the branch has to be created from

Signed-off-by: Aman Sharma <mannu.poski10@gmail.com>

* Use fully qualified name of `master`

Signed-off-by: Aman Sharma <mannu.poski10@gmail.com>

* Fix syntax

Signed-off-by: Aman Sharma <mannu.poski10@gmail.com>

* Hardcode master's SHA1 hash

Signed-off-by: Aman Sharma <mannu.poski10@gmail.com>

* Simplify pushing branch

Signed-off-by: Aman Sharma <mannu.poski10@gmail.com>

* Configure GitHub App's token

Signed-off-by: Aman Sharma <mannu.poski10@gmail.com>

* Pass token

Signed-off-by: Aman Sharma <mannu.poski10@gmail.com>

.github/workflows/combine-prs.yml

@@ -28,13 +28,21 @@ jobs:
     # The type of runner that the job will run on
     runs-on: ubuntu-latest
 
+    environment: 'Combine PR'
+
     # Steps represent a sequence of tasks that will be executed as part of the job
     steps:
+      - uses: tibdex/github-app-token@v1
+        id: generate-token
+        with:
+          app_id: ${{ secrets.APP_ID }}
+          private_key: ${{ secrets.APP_PRIVATE_KEY }}
+
       - uses: actions/github-script@v6
         id: fetch-branch-names
         name: Fetch branch names
         with:
-          github-token: ${{secrets.GITHUB_TOKEN}}
+          github-token: ${{ steps.generate-token.outputs.token }}
           script: |
             const CI_TEST_THAT_NEEDS_TO_BE_GREEN = [
               'codecov/patch',
@@ -111,8 +119,10 @@ jobs:
       - uses: actions/checkout@v2.3.3
         with:
           fetch-depth: 0
+          token: ${{ steps.generate-token.outputs.token }}
       # Creates a branch with other PR branches merged together
       - name: Created combined branch
+        id: combined-branch
         env:
           BASE_BRANCH: ${{ steps.fetch-branch-names.outputs.base-branch }}
           BRANCHES_TO_COMBINE: ${{ steps.fetch-branch-names.outputs.result }}
@@ -141,11 +151,11 @@ jobs:
         env:
           PRS_STRING: ${{ steps.fetch-branch-names.outputs.prs-string }}
         with:
-          github-token: ${{secrets.GITHUB_TOKEN}}
+          github-token: ${{ steps.generate-token.outputs.token }}
           script: |
             const prString = process.env.PRS_STRING;
             const body = 'This PR was created by the Combine PRs action by combining the following PRs:\n' + prString;
-            await github.pulls.create({
+            await github.rest.pulls.create({
               owner: context.repo.owner,
               repo: context.repo.repo,
               title: 'Combined PR',