WIP: Refactor diagnosis system

[samplise]

What changes were proposed in this pull request?

Refactor the diagnosis system according to the new design.

Why are the changes needed?

Make it simpler to implement problem diagnosis in the DLRover.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

Unit test.

[codecov]

Codecov Report

Attention: Patch coverage is 78.50467% with 23 lines in your changes missing coverage. Please review.

Project coverage is 82.72%. Comparing base (774fa01) to head (dbb4b33).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...er/python/diagnosis/diagnostician/diagnostician.py	60.00%	18 Missing ⚠️
...gnosis/diagnostician/failure_node_diagnostician.py	90.32%	3 Missing ⚠️
...lrover/python/diagnosis/common/diagnosis_action.py	90.90%	1 Missing ⚠️
.../python/elastic_agent/diagnosis/diagnosis_agent.py	94.11%	1 Missing ⚠️

❌ Your patch check has failed because the patch coverage (78.50%) is below the target coverage (85.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1475      +/-   ##
==========================================
- Coverage   82.72%   82.72%   -0.01%     
==========================================
  Files         257      259       +2     
  Lines       26314    26422     +108     
==========================================
+ Hits        21769    21858      +89     
- Misses       4545     4564      +19     

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