Refactor the miner module and add unit tests

[jiqicn]

This PR refactors the miner module to make the code more readable. Helper functions are extracted and kept in a separate module called utilities. Corresponding unit and integration tests are added, as well as the system test of the whole package.

To review this PR, one can pay attention to the following:

• If the code can be improved on coding style, quality, architecture, etc.
• If any essential tests are missing.

[sonarcloud[bot]]

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0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

95.8% Coverage
0.0% Duplication