[WIP] Add GitHub action for increasing unit test coverage

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

Original prompt

I want to increase the UT coverage of the Nydus project. Please help to add a github action that:

1. Run make coverage to get the current UT coverage of every rust files.
2. Find out the least covered file and add new UTs to increase its coverage.
3. Make sure UT passes locally
4. Run make coverage again to get the new coverage
5. Generate report on the UT increase of the file as well as the project overall.
6. Commit the change with proper commit messages and raise a pull request

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