btrfs-progs: ci: upload coverage workflow results to codecov.io

Needed to work:

- install github app Codecov, accept permissions
- copy token from codecov.io to repository secrets
- allow actions permissions to run either verified marketplace creators
  or list codecov/codecov-action@*
- set up repository as active on codecov.io, watch results eg.
  https://app.codecov.io/gh/kdave/btrfs-progs/tree/coverage-test/

Signed-off-by: David Sterba <dsterba@suse.com>

.github/workflows/coverage.yml

@@ -50,3 +50,8 @@ jobs:
         with:
           name: lcov-out
           path: lcov-out
+      - name: Upload coverage reports to Codecov
+        uses: codecov/codecov-action@v3
+        with:
+          token: ${{ secrets.CODECOV_TOKEN }}
+          files: ${{ github.workspace }}/lcov-info

README.md

@@ -3,6 +3,7 @@ Btrfs-progs
 
 [![devel](https://github.com/kdave/btrfs-progs/actions/workflows/devel.yml/badge.svg)](https://github.com/kdave/btrfs-progs/actions/workflows/devel.yml)
 [![coverage](https://github.com/kdave/btrfs-progs/actions/workflows/coverage.yml/badge.svg)](https://github.com/kdave/btrfs-progs/actions/workflows/coverage.yml)
+[![codecov](https://codecov.io/gh/kdave/btrfs-progs/branch/coverage-test/graph/badge.svg?token=fhLI8V9s0k)](https://codecov.io/gh/kdave/btrfs-progs)
 [![static](https://github.com/kdave/btrfs-progs/actions/workflows/artifacts-static-build.yml/badge.svg)](https://github.com/kdave/btrfs-progs/actions/workflows/artifacts-static-build.yml)
 [![release](https://github.com/kdave/btrfs-progs/actions/workflows/ci-build-test.yml/badge.svg)](https://github.com/kdave/btrfs-progs/actions/workflows/ci-build-test.yml)