Add workflow for drafting a new release

Drafting a new release can be quite involved. One has to update the
changelog correctly, bump the versions in the manifest files, commit
everything and raise a PR.

This workflow does all of that for you at the click of a button!

.github/workflows/draft-new-release.yml

@@ -0,0 +1,70 @@
+name: "Draft new release"
+
+on:
+  workflow_dispatch:
+    inputs:
+      version:
+        description: 'The new version in X.Y.Z format.'
+        required: true
+
+jobs:
+  draft-new-release:
+    name: "Draft a new release"
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          token: ${{ secrets.BOTTY_GITHUB_TOKEN }}
+
+      - name: Create release branch
+        run: git checkout -b release/${{ github.event.inputs.version }}
+
+      - name: Update changelog
+        uses: thomaseizinger/keep-a-changelog-new-release@1.2.1
+        with:
+          version: ${{ github.event.inputs.version }}
+          changelogPath: CHANGELOG.md
+
+      - name: Initialize mandatory git config
+        run: |
+          git config user.name "${{ secrets.BOTTY_NAME }}"
+          git config user.email ${{ secrets.BOTTY_EMAIL }}
+
+      - name: Bump version in Cargo.toml
+        uses: thomaseizinger/set-crate-version@1.0.0
+        with:
+          version: ${{ github.event.inputs.version }}
+          manifest: swap/Cargo.toml
+
+      - name: Update Cargo.lock
+        run: cargo update --workspace
+
+      - name: Commit changelog and manifest files
+        id: make-commit
+        run: |
+          curl -fsSL https://dprint.dev/install.sh | sh
+          /home/runner/.dprint/bin/dprint fmt
+
+          git add CHANGELOG.md Cargo.lock swap/Cargo.toml
+          git commit --message "Prepare release ${{ github.event.inputs.version }}"
+
+          echo "::set-output name=commit::$(git rev-parse HEAD)"
+
+      - name: Push new branch
+        run: git push origin release/${{ github.event.inputs.version }} --force
+
+      - name: Create pull request
+        uses: thomaseizinger/create-pull-request@1.0.0
+        with:
+          GITHUB_TOKEN: ${{ secrets.BOTTY_GITHUB_TOKEN }}
+          head: release/${{ github.event.inputs.version }}
+          base: master
+          title: Release version ${{ github.event.inputs.version }}
+          reviewers: ${{ github.actor }}
+          body: |
+            Hi @${{ github.actor }}!
+
+            This PR was created in response to a manual trigger of the release workflow here: https://github.com/${{ github.repository }}/actions/runs/${{ github.run_id }}.
+            I've updated the changelog and bumped the versions in the manifest files in this commit: ${{ steps.make-commit.outputs.commit }}.
+
+            Merging this PR will create a GitHub release and upload any assets that are created as part of the release build.