refactor(processor/k8s): Improve configuration and API client setup

[gfieni]

PR overview:

• Move monitor agent configuration into a dataclass.
• Build Kubernetes API client configuration explicitly for local, cluster, and manual modes.
• Move Kubernetes API client creation into the monitor agent process instead of the constructor.
• Validate manual Kubernetes API host and bearer token settings.
• Add a retry delay to prevent a tight fetch/watch/error loop when API queries fail.

[codecov]

Codecov Report

❌ Patch coverage is 70.96774% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.02%. Comparing base (496156d) to head (8879f69).

Files with missing lines	Patch %	Lines
src/powerapi/processor/pre/k8s/monitor_agent.py	63.82%	17 Missing ⚠️
src/powerapi/processor/pre/k8s/actor.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #831      +/-   ##
==========================================
- Coverage   72.12%   72.02%   -0.10%     
==========================================
  Files          90       90              
  Lines        2999     3003       +4     
==========================================
  Hits         2163     2163              
- Misses        836      840       +4     

Files with missing lines	Coverage Δ
src/powerapi/cli/generator.py	89.04% <100.00%> (+0.05%)	⬆️
...werapi/processor/pre/k8s/metadata_cache_manager.py	100.00% <100.00%> (ø)
src/powerapi/processor/pre/k8s/actor.py	84.00% <83.33%> (-3.10%)	⬇️
src/powerapi/processor/pre/k8s/monitor_agent.py	55.33% <63.82%> (-0.45%)	⬇️

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