chore: add GitHub action for stale issues and PRs (#365)

This PR enables the `notation-go` repo to run stale action at 1:30 every
day to label or close stale PRs and issues. See guideline
https://github.com/marketplace/actions/close-stale-issues

This is the definition for stale PRs or issues that we discussed during
community call and to be updated in [contributing
PR](notaryproject/.github#25).

"A stale issue is one that remains inactive or without updates for a
period of 60 days. A stale pull request (PR) is one that remains
inactive or without updates for a period of 45 days. When an issue or PR
becomes stale, it is labelled as `stale`. Normally maintainers will
comment on stale issues or PRs to prompt participants to take action. If
there is no activity for additional 30 days, this issue or PR will be
closed. If an update/comment occur on stale issues or pull requests, the
stale label will be removed, and the timer will restart"

Signed-off-by: Yi Zha <[email protected]>

.github/workflows/stale.yml

@@ -0,0 +1,33 @@
+# Copyright The Notary Project Authors.
+# 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.
+
+name: "Close stale issues and PRs"
+on:
+  schedule:
+    - cron: "30 1 * * *"
+
+jobs:
+  stale:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/stale@v8
+        with:
+          stale-issue-message: "This issue is stale because it has been opened for 60 days with no activity. Remove stale label or comment. Otherwise, it will be closed in 30 days."
+          stale-pr-message: "This PR is stale because it has been opened for 45 days with no activity. Remove stale label or comment. Otherwise, it will be closed in 30 days."
+          close-issue-message: "Issue closed due to no activity in the past 30 days."
+          close-pr-message: "PR closed due to no activity in the past 30 days."
+          days-before-issue-stale: 60
+          days-before-pr-stale: 45
+          days-before-issue-close: 30
+          days-before-pr-close: 30
+          exempt-all-milestones: true