Add CodeQL workflow (#155)

Closes #47

.github/workflows/codeql.yml

@@ -0,0 +1,60 @@
+name: "CodeQL"
+
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+    branches: [ "main" ]
+    types: [ opened, synchronize, reopened ]
+    paths-ignore:
+      - '**/*.md'
+      - '**/*.txt'
+  schedule:
+    - cron: '24 7 * * 3'  # At 07:24 on Wednesdays
+  workflow_dispatch:
+    # no inputs needed here
+
+jobs:
+  analyze:
+    name: Analyze (${{ matrix.language }})
+    runs-on: 'ubuntu-latest'
+    permissions:
+      actions: read
+      contents: read
+      security-events: write
+
+    strategy:
+      fail-fast: false
+      matrix:
+        language: [ 'java-kotlin' ]
+
+    steps:
+    - name: Checkout repository
+      uses: actions/checkout@v4
+
+    - name: Set up Java 17
+      uses: actions/setup-java@v4
+      with:
+        distribution: zulu
+        java-version: 17
+
+    - name: Initialize CodeQL
+      uses: github/codeql-action/init@v3
+      with:
+        languages: ${{ matrix.language }}
+        queries: security-extended,security-and-quality
+
+    - name: Cache Maven packages
+      uses: actions/cache@v4
+      with:
+        path: ~/.m2
+        key: ${{ runner.os }}-m2-${{ hashFiles('**/pom.xml') }}
+        restore-keys: ${{ runner.os }}-m2
+
+    - name: Autobuild
+      uses: github/codeql-action/autobuild@v3
+
+    - name: Perform CodeQL Analysis
+      uses: github/codeql-action/analyze@v3
+      with:
+        category: "/language:${{matrix.language}}"