Add coverage report by Codecov

This is needed to display coverage, and as an example of setting up
coverage counts.

Also, added badges to make the repositories look prettier.

.github/workflows/go.yml

@@ -34,6 +34,11 @@ jobs:
       - name: Teardown test environment
         run: make test-env-up
 
+      - name: Upload coverage reports to Codecov
+        uses: codecov/codecov-action@v3
+        env:
+          CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}
+
   golangci:
     name: lint
     runs-on: ubuntu-latest