Add Github Workflow for checking Go API compatibility on PRs

It'll automatically set the semver label for a given PR.
However we don't detect (yet) whether it's in actual Gophercloud code
or in the tests.

.github/workflows/semver-auto.yaml

@@ -0,0 +1,47 @@
+name: Add PR semver labels on push
+on:
+  pull_request_target:
+    types: [synchronize]
+jobs:
+  go-apidiff:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+
+    - uses: actions/setup-go@v2
+      with:
+        go-version: '1'
+
+    - name: Remove the semver label
+      uses: actions-ecosystem/action-remove-labels@2ce5d41b4b6aa8503e285553f75ed56e0a40bae0
+      with:
+        labels: |
+          semver:patch
+          semver:minor
+          semver:major
+        github_token: ${{ github.token }}
+
+    - name: Checking Go API Compatibility
+      id: go-apidiff
+      uses: joelanford/go-apidiff@v0.7.0
+
+    - name: Add semver:patch label
+      if: always() && steps.go-apidiff.outputs.semver-type == 'patch'
+      uses: actions-ecosystem/action-add-labels@bd52874380e3909a1ac983768df6976535ece7f8
+      with:
+        github_token: ${{ secrets.GITHUB_TOKEN }}
+        labels: semver:patch
+
+    - name: Add semver:minor label
+      if: always() && steps.go-apidiff.outputs.semver-type == 'minor'
+      uses: actions-ecosystem/action-add-labels@bd52874380e3909a1ac983768df6976535ece7f8
+      with:
+        github_token: ${{ secrets.GITHUB_TOKEN }}
+        labels: semver:minor
+
+    - name: Add semver:major label
+      if: always() && steps.go-apidiff.outputs.semver-type == 'major'
+      uses: actions-ecosystem/action-add-labels@bd52874380e3909a1ac983768df6976535ece7f8
+      with:
+        github_token: ${{ secrets.GITHUB_TOKEN }}
+        labels: semver:major