feat: support multiple reporters at once + make reporters and github details configurable from config file, env variables, CLI flags

[leodido]

This PR does many important things. Just to cite the 3 most important ones:

• promotes the reporter and its related flags to ConfigFlags (fixes Make the --reporter flag and the GitHub flags related to it configurable via env and via config file #175)
  • it will be possible to define them via the .lstn.yaml (or another) config file
  • it will be possible from now on to define them via ENV VARIABLES
• defines the reporters as enums (fixes Define the reporters as enum values and use those as flag values #174)
  • at the moment we have support for:
    • gh-pull-comment (implemented)
    • gh-pull-review (TBD)
    • gh-pull-check (TBD report scan PR github check  #167)
• allows users to run 1 or more reporters above at once (fixes Allow users to run 1 or more reporters at once #173)

What's missing?

• [ ] Contextual validators on reporters and GitHub flags (eg., --gh-pull-id mandatory if --reporter, etc. etc.) (Contextual validation of reporter flags #172)

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• I have read the contributing guidelines
• I have followed the coding guidelines
• I have written unit tests
• I have made sure that the pull request is of reasonable size and can be easily reviewed

[fntlnz]

Incredible work, flags are much more consistent now and the ability to have many reporters is just so needed. Thank you

LGTM