feat: make config monkeypatchable, fix config related startup issues

[CarliJoy]

Make configuration monkeypatchable.

Also show users how within readme.

This should also fix the bug in #830 and supersedes #821 and #832

[codecov]

Codecov Report

Attention: Patch coverage is 97.56098% with 1 line in your changes missing coverage. Please review.

Please upload report for BASE (main@632e5f4). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
core/testcontainers/core/config.py	97.29%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #833   +/-   ##
=======================================
  Coverage        ?   85.71%           
=======================================
  Files           ?       13           
  Lines           ?      707           
  Branches        ?      115           
=======================================
  Hits            ?      606           
  Misses          ?       76           
  Partials        ?       25           

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