Add some type annotations

[kulisak12]

Changes in .pyi files are to make mypy happy.

#6863

[codecov[bot]]

Codecov Report

:x: Patch coverage is 95.03546% with 7 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 81.05%. Comparing base (a329890) to head (a9dff13). :warning: Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
src/borg/helpers/datastruct.py	81.81%	0 Missing and 4 partials :warning:
src/borg/cache.py	95.52%	3 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9094      +/-   ##
==========================================
- Coverage   81.07%   81.05%   -0.02%     
==========================================
  Files          77       77              
  Lines       13540    13568      +28     
  Branches     2008     2014       +6     
==========================================
+ Hits        10978    10998      +20     
- Misses       1900     1903       +3     
- Partials      662      667       +5     

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