feat: Add workflow trigger for incident called 'alert_association_changed'

[dvora-ns]

Closes #5253

📑 Description

This PR adds a new workflow trigger event for incident called "alert_association_changed". This helps user configure workflows that trigger whenever incident alert association changes.

Following events cause the incident alert association to change:

1. Alert addition/linkage to an incident
2. Alert unlinking from an incident.

Whenever this event takes place, an additional key called "linked_alerts" is added to the incident object that's passed to the workflow. The linked_alerts key is basically a list of alert strings currently linked to the incident sorted in descending order of timestamp. Each alert string present in the list is of the following form: {status} {lastReceived} [{severity}] {description} e.g. Firing 2025-01-30T10:00:00.000Z [high] Test alert 1 description

✅ Checks

• [x] My pull request adheres to the code style of this project
• [ ] My code requires changes to the documentation
• [ ] I have updated the documentation as required
• [ ] All the tests have passed

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[codecov[bot]]

Codecov Report

:x: Patch coverage is 0% with 17 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 30.70%. Comparing base (74e4c9f) to head (d7f7b2f). :warning: Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
keep/workflowmanager/workflowmanager.py	0.00%	12 Missing :warning:
keep/api/bl/incidents_bl.py	0.00%	4 Missing :warning:
keep/rulesengine/rulesengine.py	0.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5254      +/-   ##
==========================================
- Coverage   30.73%   30.70%   -0.04%     
==========================================
  Files         101      101              
  Lines       11494    11511      +17     
==========================================
+ Hits         3533     3534       +1     
- Misses       7961     7977      +16     

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