Shared plugin configs for multi-group plugins

[achimnol]

This PR makes it possible to use "config/plugins/_/<entrypoint-name>" as the place for shared
configs for multi-group plugins (e.g., plugins that have both hook and webapp plugins).

• Read and merge configs from the shared location and group-sepcific location
• Unify the watcher to monitor the prefix changes and dispatch the changes to plugins

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (a86c9f7) 75.24% compared to head (34f2c15) 75.24%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #40   +/-   ##
=======================================
  Coverage   75.24%   75.24%           
=======================================
  Files          17       17           
  Lines        2375     2375           
=======================================
  Hits         1787     1787           
  Misses        588      588           

Impacted Files	Coverage Δ
src/ai/backend/common/plugin/__init__.py	90.69% <100.00%> (ø)

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