Validate mono changesets in CI

Add changeset validation with `mono changeset validate` as a
reusable workflow, and reference it from the CI and publish
release workflows.

Author: unflxw

.github/workflows/ci.yml

@@ -104,3 +104,6 @@ jobs:
         with:
           fetch-depth: 0 # Fetch depth is required
       - uses: lintje/action@v0.11
+
+  validate-changesets:
+    uses: ./.github/workflows/validate_changesets.yml

.github/workflows/validate_changesets.yml

@@ -0,0 +1,24 @@
+name: Validate changesets
+
+on:
+  workflow_call:
+
+jobs:
+  validate-changesets:
+    name: Validate changesets
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout project
+        uses: actions/checkout@v4
+      - name: Checkout Mono
+        uses: actions/checkout@v4
+        with:
+          repository: appsignal/mono
+          path: tmp/mono
+      - name: Install Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: '3.3'
+      - name: Validate changesets
+        run: tmp/mono/bin/mono changeset validate
+