build: add cd GitHub workflow

Add GitHub workflow to build securesystemslib and release on GitHub and
PyPI. The workflow is copied from python-tuf with minimal changes, see:

```
diff \
  <(curl -s https://raw.githubusercontent.com/theupdateframework/python-tuf/c92cd28b38d3af5ee24411a3c2082fc2d6c37f4b/.github/workflows/cd.yml) \
  .github/workflows/cd.yml
```

Prerequisites and usage details are described at
https://github.com/theupdateframework/python-tuf/blob/v3.1.0/docs/RELEASE.md.
(a verify_release script does not exist for securesystemslib)

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

.github/workflows/cd.yml

@@ -0,0 +1,118 @@
+name: CD
+concurrency: cd
+
+on:
+  push:
+    tags:
+      - v*
+
+permissions: {}
+
+jobs:
+  test:
+    name: Test
+    uses: ./.github/workflows/_test.yml
+
+  build:
+    name: Build
+    runs-on: ubuntu-latest
+    needs: test
+    steps:
+      - name: Checkout release tag
+        uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11 # v4.1.1
+        with:
+          ref: ${{ github.event.workflow_run.head_branch }}
+
+      - name: Set up Python
+        uses: actions/setup-python@65d7f2d534ac1bc67fcd62888c5f4f3d2cb2b236 # v4.7.1
+        with:
+          python-version: '3.x'
+
+      - name: Install build dependency
+        run: python3 -m pip install -r requirements-build.txt
+
+      - name: Build binary wheel and source tarball
+        run: python3 -m build --sdist --wheel --outdir dist/ .
+
+      - name: Store build artifacts
+        uses: actions/upload-artifact@a8a3f3ad30e3422c9c7b888a15615d19a852ae32 # v3.1.3
+        # NOTE: The GitHub release page contains the release artifacts too, but using
+        # GitHub upload/download actions seems robuster: there is no need to compute
+        # download URLs and tampering with artifacts between jobs is more limited.
+        with:
+          name: build-artifacts
+          path: dist
+
+  candidate_release:
+    name: Release candidate on Github for review
+    runs-on: ubuntu-latest
+    needs: build
+    permissions:
+      contents: write # to modify GitHub releases
+    outputs:
+      release_id: ${{ steps.gh-release.outputs.result }}
+    steps:
+      - name: Fetch build artifacts
+        uses: actions/download-artifact@9bc31d5ccc31df68ecc42ccf4149144866c47d8a # v3.0.2
+        with:
+          name: build-artifacts
+          path: dist
+
+      - id: gh-release
+        name: Publish GitHub release draft
+        uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea # v7.0.1
+        with:
+          script: |
+            fs = require('fs')
+            res = await github.rest.repos.createRelease({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              name: '${{ github.ref_name }}-rc',
+              tag_name: '${{ github.ref }}',
+              body: 'Release waiting for review...',
+            });
+
+            fs.readdirSync('dist/').forEach(file => {
+              github.rest.repos.uploadReleaseAsset({
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                release_id: res.data.id,
+                name: file,
+                data: fs.readFileSync('dist/' + file),
+              });
+            });
+            return res.data.id
+
+  release:
+    name: Release
+    runs-on: ubuntu-latest
+    needs: candidate_release
+    environment: release
+    permissions:
+      contents: write # to modify GitHub releases
+      id-token: write # to authenticate as Trusted Publisher to pypi.org
+    steps:
+      - name: Fetch build artifacts
+        uses: actions/download-artifact@9bc31d5ccc31df68ecc42ccf4149144866c47d8a # v3.0.2
+        with:
+          name: build-artifacts
+          path: dist
+
+      - name: Publish binary wheel and source tarball on PyPI
+        # Only attempt pypi upload in upstream repository
+        if: github.repository == 'secure-systems-lab/securesystemslib'
+        uses: pypa/gh-action-pypi-publish@b7f401de30cb6434a1e19f805ff006643653240e # v1.8.10
+
+      - name: Finalize GitHub release
+        uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea # v7.0.1
+        with:
+          script: |
+            github.rest.repos.updateRelease({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              release_id: '${{ needs.candidate_release.outputs.release_id }}',
+              name: '${{ github.ref_name }}',
+              body: 'See [CHANGELOG.md](https://github.com/' +
+                     context.repo.owner + '/' + context.repo.repo +
+                    '/blob/${{ github.ref_name }}/CHANGELOG.md) for details.'
+            })