Add GitHub workflow for code coverage calculation (ical-org#624)

* Add GitHub workflow for code coverage calculation

At the moment the workflow is "on demand". Should become part of the test workflow, if everything works fine.
Final workflow code is commented out.

* Enable trigger for pull request and push to main

* Add code coverage configuration file for Codecov

.github/workflows/coverage.yml

@@ -0,0 +1,49 @@
+name: Code Coverage
+# This job runs unit tests and generates a code coverage report.
+# The report is stored as artifact, and uploaded to Codecov.
+
+on:
+  # workflow_dispatch: {}  # temporarily enable the workflow for manual runs
+  pull_request:
+    branches:
+      - main
+  push:
+    branches:
+      - main
+
+jobs:
+  coverage:
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Checkout repository
+      uses: actions/checkout@v4
+      with:
+        fetch-depth: 0  # Fetch all history for all tags and branches
+    - name: Setup .NET
+      uses: actions/setup-dotnet@v4
+      with:
+        dotnet-version: |
+          8.0.x
+    - name: Restore dependencies --verbosity quiet
+      run: dotnet restore
+    - name: Add AltCover package to test project
+      run: dotnet add ./Ical.Net.Tests/Ical.Net.Tests.csproj package AltCover
+
+    - name: Build
+      run: dotnet build --no-restore --configuration Release -p:nowarn=1591
+    - name: Test
+      run: dotnet test --no-build --configuration Release --verbosity quiet /p:AltCover=true /p:AltCoverXmlReport="coverage.xml" /p:AltCoverStrongNameKey="../IcalNetStrongnameKey.snk" /p:AltCoverAssemblyExcludeFilter="Ical.Net.Tests|NUnit3.TestAdapter|AltCover" /p:AltCoverAttributeFilter="ExcludeFromCodeCoverage" /p:AltCoverLineCover="false"
+
+    - name: Store coverage report as artifact
+      uses: actions/upload-artifact@v4
+      with:
+        name: coverage-report
+        path: ./Ical.Net.Tests/coverage.*.xml  # store all coverage reports
+    - name: Upload coverage to Codecov  
+      uses: codecov/codecov-action@v4  
+      with:  
+        # files: automatically finds all in ./Ical.Net.Tests/
+        name: coverage
+        token: ${{ secrets.CODECOV_TOKEN }}
+        fail_ci_if_error: false

codecov.yml

@@ -0,0 +1,17 @@
+coverage:
+  precision: 0      # no decimals
+  round: nearest    # round up/down
+  range: "80...100"
+
+  status:
+    project:        # show status on project level
+      default:
+        target: 80  # Set the target coverage percentage
+    patch:          # show status on patch level
+      default:
+        target: 80  # minimum coverage for successful commit 
+
+comment:
+  layout: "reach, diff, flags, files"
+  behavior: default
+  require_changes: true  # Only comment if there are changes in coverage