ci: add Codecov to check test coverage

.github/workflows/codecov.yml

@@ -0,0 +1,26 @@
+name: Codecov CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+jobs:
+  codecov:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v4
+
+    - name: Set up Go
+      uses: actions/setup-go@v4
+      with:
+        go-version: "1.21"
+
+    - name: Generate coverage report ./...
+      run: go test -coverprofile=coverage.txt
+
+    - name: Upload coverage reports to Codecov
+      uses: codecov/codecov-action@v3
+      env:
+        CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}

.gitignore

@@ -3,3 +3,4 @@
 /bats
 /cmd/hcloud/hcloud
 hcloud_cli.p12
+coverage.txt