fix: add PEP 561 py.typed markers for type checker support

[andres-ito-traversal]

Add py.typed marker files to core and all module packages to indicate type information is available. This enables type checkers like Pyright and mypy to recognize and validate type hints in testcontainers packages.

Resolves "Stub file not found" errors when running type checkers on code that imports testcontainers modules.

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 79.52%. Comparing base (5853d32) to head (9955664).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #923      +/-   ##
==========================================
- Coverage   79.78%   79.52%   -0.26%     
==========================================
  Files          14       14              
  Lines        1182     1182              
  Branches      184      184              
==========================================
- Hits          943      940       -3     
- Misses        197      201       +4     
+ Partials       42       41       -1     

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