Implement added_conftest_path

[segoon]

Add an argument for extra conftest files paths. It is used in specific environments where fixtures are collected through nonstandard way, e.g. in Yandex monorepository Arcadia. One may define a list of extra conftest.py files and use fixtures from there.

Also one can define pytest plugins in 'pytest_plugins' local variable and expect fixtures from the imported plugins to be accessible via jedi.

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (f8e502f) 94.50% compared to head (60b10a4) 94.44%.

❗ Current head 60b10a4 differs from pull request most recent head 7528db7. Consider uploading reports for the commit 7528db7 to get more accurate results

Files	Patch %	Lines
jedi/plugins/pytest.py	77.77%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1976      +/-   ##
==========================================
- Coverage   94.50%   94.44%   -0.07%     
==========================================
  Files          80       80              
  Lines       11917    11930      +13     
==========================================
+ Hits        11262    11267       +5     
- Misses        655      663       +8     

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[davidhalter]

Sorry, but this is something that we won't support (due to the ongoing maintenance). In general I'm avoiding such special cases. Thanks for the attempt, though.