Add custom scenarios dir

[tsv1]

Resolves #112

Add support for specifying a custom directory for Vedro scenarios to improve project structure flexibility.

[codecov]

Codecov Report

Attention: Patch coverage is 67.81609% with 28 lines in your changes missing coverage. Please review.

Project coverage is 90.72%. Comparing base (86bfcf8) to head (6088782).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
vedro/commands/run_command/_run_command.py	61.53%	18 Missing and 2 partials ⚠️
vedro/plugins/skipper/_discoverer.py	20.00%	4 Missing ⚠️
vedro/plugins/skipper/_skipper.py	81.25%	2 Missing and 1 partial ⚠️
...edro/core/scenario_loader/_scenario_file_loader.py	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #116      +/-   ##
==========================================
- Coverage   91.02%   90.72%   -0.31%     
==========================================
  Files         165      165              
  Lines        4681     4742      +61     
  Branches      655      672      +17     
==========================================
+ Hits         4261     4302      +41     
- Misses        364      381      +17     
- Partials       56       59       +3     

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