fix(server): add missing username to auth as premium user

[mdevolde]

fix(server): add missing username to auth as premium user

Why the pull request was made

To patch the "premium" functionality, and allow users to authenticate as premium users to LT public API.

Summary of changes

• Add a new premium_username attribute to LanguageTool.
• Set this attribute in the check request params.
• Adapt existing documentation.

Screenshots (if appropriate):

Not applicable.

How has this been tested?

Applied local tests. Need a premium key to test the functionality.

Resources

https://languagetool.org/http-api/

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update (changes to documentation only)
• Refactor / code style update (non-breaking change that improves code structure or readability)
• Tests / CI improvement (adding or updating tests or CI configuration only)
• Other (please describe):

Checklist

• Followed the project's contributing guidelines.
• Updated any relevant tests.
• Updated any relevant documentation.
• Added comments to your code where necessary.
• Formatted your code, run the linters, checked types and tests.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 75.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.46%. Comparing base (ea04de4) to head (16b1335).

Files with missing lines	Patch %	Lines
src/language_tool_python/server.py	75.00%	3 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #198      +/-   ##
==========================================
- Coverage   80.51%   80.46%   -0.05%     
==========================================
  Files          13       13              
  Lines        1735     1746      +11     
==========================================
+ Hits         1397     1405       +8     
- Misses        338      341       +3     

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