Add --gpt-prompt flag to provide a prompt ready to be used with a GPT engine

[jordilin]

The --gpt-prompt flag will be used to automatically create a prompt that
can be used with GPT engines to create a title and a description for the
outgoing commit messages.

[codecov]

Codecov Report

Attention: Patch coverage is 20.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 85.94%. Comparing base (4738abd) to head (598ee30).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/cmds/merge_request.rs	0.00%	3 Missing ⚠️
src/cli/merge_request.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #314      +/-   ##
==========================================
- Coverage   85.96%   85.94%   -0.02%     
==========================================
  Files          69       69              
  Lines       16982    16987       +5     
  Branches    16982    16987       +5     
==========================================
+ Hits        14598    14599       +1     
- Misses       2196     2200       +4     
  Partials      188      188              

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