Add plugins.py module with load_plugin_on_server helper

[PProfizi]

This PR proposes a new module called plugins aimed at management of DPF Python plugins.

Here the helper load_plugin_on_server allows to automate the steps required to send a Python plugin server-side and load it correctly.

It is based on the idea that DPF Python plugins are shipped as Python libraries to install in the active Python virtual environment.

[codecov]

Codecov Report

Attention: Patch coverage is 24.00000% with 19 lines in your changes are missing coverage. Please review.

Project coverage is 81.92%. Comparing base (a6943c2) to head (f43c0f6).
Report is 10 commits behind head on master.

❗ Current head f43c0f6 differs from pull request most recent head 15e7539

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1600      +/-   ##
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+ Coverage   80.12%   81.92%   +1.79%     
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  Files          82       83       +1     
  Lines        9671     9721      +50     
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+ Hits         7749     7964     +215     
+ Misses       1922     1757     -165