Automatic oauth2 token retrieval

[dtrai2]

Adds the possibility to automatically retrieve access tokens from one running keycloak instance.

ToDos:

• implement feature
• clean up implementation
• add documentation
• add tests

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 99.12281% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 92.54%. Comparing base (31188ed) to head (bb63c92).
Report is 2 commits behind head on main.

Files	Patch %	Lines
logprep/util/credentials.py	99.04%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #531      +/-   ##
==========================================
+ Coverage   92.34%   92.54%   +0.20%     
==========================================
  Files         144      145       +1     
  Lines       10041    10233     +192     
==========================================
+ Hits         9272     9470     +198     
+ Misses        769      763       -6     

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