Github: PR Labeler action (#972)

Stealing this from Agones, this GitHub action will label PRs based on
the PR template, to make life a bit easier when doing releases, as
labels will already be applied (if the template is used).

Also removed the `/kind hotfix` since that never applied to this project
anyway.

.github/pull_request_template.md

@@ -14,7 +14,6 @@
 > /kind cleanup
 > /kind documentation
 > /kind feature
-> /kind hotfix
 
 **What this PR does / Why we need it**:
 

.github/workflows/label-pr.yml

@@ -0,0 +1,68 @@
+---
+# Copyright 2023 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+# This workflow utilizes https://github.com/actions/github-script in GitHub Actions to apply labels to pull requests.
+#
+name: Label PR
+on: [pull_request_target]
+jobs:
+  label:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      pull-requests: write
+    steps:
+      - name: Label PR
+        uses: actions/github-script@v6
+        with:
+          script: |-
+            const keywords = {
+              "breaking": "kind/breaking",
+              "bug": "kind/bug",
+              "feature": "kind/feature",
+              "cleanup": "kind/cleanup",
+              "documentation": "kind/documentation",
+              "release": "kind/release"
+            };
+            const prBody = context.payload.pull_request.body;
+            const prLabels = [];
+            if (prBody === null || prBody.trim() === "") {
+              console.log("Pull Request body is empty");
+              prLabels.push("kind/other");
+            } else {
+              const regex = /^\s*\/kind\s+(.+)$/m;
+              const match = prBody.match(regex);
+              console.log(`PR body: '${prBody}'`);
+              console.log(`Regex match: '${match}'`);
+              if (match && match[1] in keywords) {
+                const keyword = match[1];
+                const label = keywords[keyword];
+                console.log(`Adding label: '${label}' based on keyword '${keyword}'`);
+                prLabels.push(label);
+              } else {
+                console.log(`Adding label: 'kind/other' as no matching keyword found.`);
+                prLabels.push("kind/other");
+              }
+            }
+            try {
+              github.rest.issues.addLabels({
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                issue_number: context.payload.pull_request.number,
+                labels: prLabels
+              });
+            } catch (error) {
+              console.error(`Error retrieving files: ${error}`);
+            }