refactor: AppInformer now uses GenericInformer as foundation

[jannfis]

This refactor makes the AppInformer use GenericInformer instead of rolling its own.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 65.00000% with 42 lines in your changes missing coverage. Please review.

Project coverage is 46.91%. Comparing base (97e0bdd) to head (5bdcbb5).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/informer/application/appinformer.go	64.83%	24 Missing and 8 partials ⚠️
agent/agent.go	40.00%	2 Missing and 1 partial ⚠️
internal/informer/application/options.go	76.92%	3 Missing ⚠️
principal/server.go	25.00%	2 Missing and 1 partial ⚠️
...ernal/backend/kubernetes/application/kubernetes.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #221      +/-   ##
==========================================
+ Coverage   46.83%   46.91%   +0.07%     
==========================================
  Files          57       57              
  Lines        4951     4896      -55     
==========================================
- Hits         2319     2297      -22     
+ Misses       2451     2420      -31     
+ Partials      181      179       -2     

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