feat: Improve handling of pre-processors with missing dependencies

[gfieni]

This PR improves how pre-processors with missing external dependencies are handled.

Detailed changes :

• Let ImportError propagate from pre-processors modules.
• Remove eager pre-processors exports from powerapi.processor module.
• Update CLI processor factories to use lazy imports so missing dependencies are handled at configuration time rather than module import time.
• Cleanup CLI processor generator factories initialization (keep it similar to other generators)
• Rework unit tests to isolate optional-processors behavior :
  • Add OpenStack generator unit tests.
  • Split generator tests by platform. (with automatic skip when dependencies are missing)
  • Use pre-processor without external dependencies for generic tests.
  • Rework binding manager tests to work without external dependencies.

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.70%. Comparing base (67beec6) to head (2df2f7c).
⚠️ Report is 13 commits behind head on master.

Files with missing lines	Patch %	Lines
src/powerapi/cli/generator.py	88.88%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #797      +/-   ##
==========================================
+ Coverage   70.64%   70.70%   +0.06%     
==========================================
  Files          90       88       -2     
  Lines        2892     2895       +3     
==========================================
+ Hits         2043     2047       +4     
+ Misses        849      848       -1     

Files with missing lines	Coverage Δ
src/powerapi/cli/generator.py	89.30% <88.88%> (+2.15%)	⬆️

... and 2 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.