feat: post comments using action

[vladopajic]

currently go-test-coverage doesn't post comments on github PRs. this functionality is implemented using other actions (as explained here).

this issue tracks effort to implement posting comments on github PRs using this action.

for example when comment-report: true input is used github action can post/update comment with report directly on PR without need for other actions.

code example:

 - name: check test coverage
    uses: vladopajic/go-test-coverage@v2
    with:
      config: ./.github/.testcoverage.yml
      profile: ubuntu-latest-profile,macos-latest-profile,windows-latest-profile
      comment-report: true