feat: inject additional MAPDL command line arguments through an env var

[germa89]

Description

Add PYMAPDL_ADDITIONAL_SWITCHES env var to allow injecting additional switches in PyMAPDL launcher.

Issue linked

NA, but this is an issue raised in #3805 and in docker container builder repo.

Checklist

• I have visited and read Develop PyMAPDL.
• I have tested my changes locally
• I have added necessary documentation or updated existing documentation.
• I have followed the PyMAPDL coding style guidelines.
• I have added appropriate unit tests that also ensure the minimal coverage criteria.
• I have reviewed my changes before submitting this pull request.
• I have linked the issue or issues that are solved to the PR if any.
• I have assigned this PR to myself.
• I have marked this PR as draft if it is not ready to be reviewed yet.
• I have made sure that the title of my PR follows Commit naming conventions (e.g. feat: adding new MAPDL command)

[ansys-reviewer-bot]

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[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.39%. Comparing base (16508b7) to head (615934a).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3817      +/-   ##
==========================================
- Coverage   88.44%   88.39%   -0.05%     
==========================================
  Files         187      187              
  Lines       14751    14760       +9     
==========================================
+ Hits        13046    13047       +1     
- Misses       1705     1713       +8     

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[germa89]

@pyansys-ci-bot LGTM.

[pyansys-ci-bot]

✅ Approving this PR because germa89 said so in here 😬