feat(ci): add commits check to CI

Closes bitcoindevkit#1119.
Creates `commits.yml` in `.github/workflows/` directory with a new GH Action
that has 2 jobs:

1. `signed-commits`: uses [`1Password/check-signed-commits-action`](https://github.com/1Password/check-signed-commits-action)
   to check if all commits in the PR are signed.
1. `conventional-commits`: uses [`aevea/commitsar`](https://github.com/aevea/commitsar)
   to check if all commits in the PR follows the conventional commits](https://www.conventionalcommits.org/en/v1.0.0-beta.2/),
   i.e. they are prefixed with `fix`, `refactor`, `feat` etc.

Author: realeinherjar

.github/workflows/commits.yml

@@ -0,0 +1,21 @@
+name: Check Commits
+
+on:
+  pull_request:
+    branches: master
+
+jobs:
+  signed-commits:
+    name: Check signed commits in PR
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Check signed commits in PR
+        uses: 1Password/check-signed-commits-action@v1
+  conventional-commits:
+    name: Conventional Commits
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Check conventional commits in PR
+        uses: aevea/commitsar@v0.20