ci: Add support for CLA assistant

.github/workflows/cla.yml

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+name: "CLA Assistant"
+on:
+  issue_comment:
+    types: [created]
+  pull_request_target:
+    types: [opened, closed, synchronize]
+
+# Explicitly configure permissions, in case your GITHUB_TOKEN workflow permissions are set to read-only in repository settings
+permissions:
+  actions: write
+  contents: write
+  pull-requests: write
+  statuses: write
+
+jobs:
+  cla-assistant:
+    runs-on: ubuntu-latest
+    steps:
+      - name: "CLA Assistant"
+        if: (github.event.comment.body == 'recheck' || github.event.comment.body == 'I have read the CLA document and I hereby sign the CLA.') || github.event_name == 'pull_request_target'
+        uses: Holzhaus/cla-assistant-github-action@master
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          # The below token should have repo scope and must be manually added by you in the repository's secret
+          # This token is required only if you have configured to store the signatures in a remote repository/organization
+          PERSONAL_ACCESS_TOKEN: ${{ secrets.CLA_TOKEN }}
+        with:
+          # Location of the signature file.
+          remote-organization-name: Holzhaus
+          remote-repository-name: mixxx-cla
+          branch: "main"
+          path-to-signatures: "cla.json"
+          # URL of the CLA document.
+          path-to-document: "https://github.com/Holzhaus/mixxx/blob/master/CONTRIBUTOR_AGREEMENT.md"
+          # Name of users that do not have to sign a CLA.
+          allowlist: "*[bot]"
+
+          # The following are the optional inputs - If the optional inputs are not given, then default values will be taken
+          #custom-notsigned-prcomment: 'pull request comment with Introductory message to ask new contributors to sign'
+          custom-notsigned-prcomment: |
+            Thank you for your submission, we really appreciate it. :+1:
+            As a first-time contributor, we need you to sign the [Mixxx Contributor Agreement](https://github.com/Holzhaus/mixxx/blob/master/CONTRIBUTOR_AGREEMENT.md).
+            This gives us permission to distribute your contribution under the GNU Public License 2.0 or later *and* in the Apple Mac App Store.
+
+            You can sign the CLA by just posting a comment with the text below.
+
+          custom-pr-sign-comment: "I have read the CLA document and I hereby sign the CLA."
+          custom-allsigned-prcomment: "Great, all contributors have signed the CLA. :tada:"
+          # Lock PR after merging to prevent contributors from revoking their signature comments after merge.
+          lock-pullrequest-aftermerge: true

CONTRIBUTOR_AGREEMENT.md

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+# Mixxx Contributor Agreement
+
+Thank you for contributing to the Mixxx project!
+
+In order to distribute your changes with Mixxx, we need a record of your permission to distribute them under Mixxx's license.
+
+Mixxx is distributed under the GNU General Public License Version 2 (the GPLv2).
+The full text of the GPLv2 can be found at [1].
+Because a significant number of Mixxx downloads are through the Mac OS X App Store, we also license Mixxx under the GPLv2 with a special "App Store Exception".
+For more details on GPL exceptions, see [2].
+
+The text of the Mixxx App Store Exception is as follows:
+
+> (13.) In addition, as a special exception, you are granted the additional
+> right to distribute this Program subject to the Mac App Store Product
+> Usage Rules and Licensed Application End User License Agreement,
+> if the source code is made available on the Mixxx Website under the
+> terms of the GPL.
+
+By submitting this form, you agree to the following terms for your present and future contributions to Mixxx:
+
+* You retain the copyright on your contribution but give Mixxx irrevocable permission to distribute your contribution under the above license. We will contact you to obtain your permission to change the license terms of your contributions. You agree that if we cannot reach you within 30 days by the email address you provide below then you permit Mixxx to re-license your contributions without your explicit permission.
+* You represent that you are legally entitled to grant the above license. If your employer has the rights to your contributions, you represent that you have permission from your employer to grant the above license.
+* You represent that each of your contributions is your original creation.
+* You represent that your contribution includes complete details of any third-party license or other restriction (including, but not limited to, related patents and trademarks) of which you are personally aware and which are associated with any part of contributions.
+* You grant to Mixxx and to recipients of Mixxx a perpetual, worldwide, non-exclusive, no-charge, royalty-free, irrevocable (except as stated in this section) patent license to make, have made, use, offer to sell, sell, import, and otherwise transfer your contributions.
+* Your contribution is provided without warranty of any kind.
+
+[1]: http://www.gnu.org/licenses/gpl-2.0.html
+[2]: http://www.gnu.org/licenses/old-licenses/gpl-2.0-faq.html