[ci] Support to merge SONiC package versions change pr automatically (#7065)

Support to merge the SONiC package versions upgrade pull request automatically.

#### How I did it
Add the automerge tag in the pull request.
1. Sent by the author mssonicbld
2. Only contains changes in files/build/**/*
3. Only for the specified branch 202012

Merge the pull request.
1. Sent by the author mssonicbld
2. The Azure Pipelines checker complete and successful
3. The pull request contains label automerge

.github/labeler.yml

@@ -0,0 +1,2 @@
+automerge:
+- all: ['files/build/versions/**']

.github/workflows/automerge.yml

@@ -0,0 +1,22 @@
+# This workflow will merge pull requests automatically
+#
+
+name: automerge
+on:
+  check_suite:
+    types:
+      - completed
+
+jobs:
+  automerge:
+    if: github.event.check_suite.app.name == 'Azure Pipelines' &&  github.event.check_suite.conclusion == 'success'
+    runs-on: ubuntu-latest
+    steps:
+    - name: automerge
+      uses: 'pascalgn/automerge-action@v0.13.1'
+      env:
+        GITHUB_TOKEN: '${{ secrets.GITHUB_TOKEN }}'
+        MERGE_LABELS: 'automerge'
+        MERGE_METHOD: 'squash'
+        MERGE_FILTER_AUTHOR: 'mssonicbld'
+        MERGE_DELETE_BRANCH: true

.github/workflows/label.yml

@@ -0,0 +1,25 @@
+# This workflow will triage pull requests and apply a label based on the
+# paths that are modified in the pull request.
+#
+# To use this workflow, you will need to set up a .github/labeler.yml
+# file with configuration.  For more information, see:
+# https://github.com/actions/labeler
+
+name: Labeler
+on:
+  pull_request:
+    types:
+    - opened
+    - reopened
+    branches:
+    - '202012'
+    paths:
+    - 'files/build/versions/**'
+
+jobs:
+  label:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/labeler@main
+      with:
+        repo-token: "${{ secrets.GITHUB_TOKEN }}"