feat(ci): add Codecov coverage report upload

Add a step to the CI workflow that uploads coverage reports to Codecov.
This step uses the `codecov/codecov-action@v3` action and requires the `CODECOV_TOKEN` secret.

Author: phodal

.github/workflows/build.yml

@@ -103,6 +103,11 @@ jobs:
         with:
           files: ${{ github.workspace }}/build/reports/kover/xml/report.xml
 
+      - name: Upload coverage reports to Codecov
+        uses: codecov/codecov-action@v3
+        env:
+          CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}
+
       # Run tests
       - name: Build Plugin
         run: ./gradlew :plugin:buildPlugin