Add report formatter for Github Actions annotations

Is your feature request related to a problem? Please describe.
It may be useful to have a simple formatter compatible with Github Actions, in order to use it for CI jobs.

Describe the solution you'd like
The CLI shoud return a text formatted in a way that is compatible with Github Actions annotations fo workflows: https://docs.github.com/en/actions/reference/workflows-and-actions/workflow-commands#about-workflow-commands

This is an example from pylint

************* Module src.MyClass                                                                                                                                                                                         
::warning file=app/src/MyClass.py,line=22,endline=22,col=8,title=W1203::Use lazy % formatting in logging functions

Describe alternatives you've considered
The official Github Action requires Advanced Security that is a paid addon also for paying teams for private repositories.

The custom formatter is not suitable, because the Github Actions annotations require a multiline structure.

Additional context
Pylint has a dedicated output formatter for Github actions: https://pylint.pycqa.org/en/latest/user_guide/usage/output.html (--output-format=github).

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Start at Bandit's CLI formatter entry point and inspect the custom formatter path alongside existing output formats. Add a selectable GitHub Actions format matching the multiline annotation example, then verify the CLI output against that example with formatter tests.