feat(server): Add ServerContext.xml_path setter

[PProfizi]

To allow setting the xml_path for a dpf.AvailableServerContexts.custom_defined context.

First remark: the right AvailableServerContext for using a custom xml path is no_context (context_type=2) instead of custom_defined (context_type=3)... this is confusing.

Second remark: despite the tests passing on my local machine with a unified Ansys install, it does not pass in the GH pipelines with a standalone DPF. Is there something I am missing?

@rafacanton @ansys-akarcher I think I will need some help on this one. This comes from a workaround shared to a client where they needed to remove multiphysics_mapper from default plugins. It is easier for them to share a custom XML file and use it within a PyDPF script rather than modifying their actual DPF installation.
The workaround shared was to use ServerContext(context_type=2, xml_path="path/to/file").
The point here is to make this user-friendly.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 46.98%. Comparing base (7c5a4c4) to head (e3d18a1).

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #2386       +/-   ##
===========================================
- Coverage   84.20%   46.98%   -37.23%     
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  Files          91       91               
  Lines       10675    10679        +4     
===========================================
- Hits         8989     5017     -3972     
- Misses       1686     5662     +3976     

[github-actions]

The documentation for this pull request will be available at https://dpf.docs.pyansys.com/pull/2386. Please allow some time for the documentation to be deployed.