Add audit log field to RetrieveEYBLeadSerializer

[oliverjwroberts]

Description of change

This PR refactors audit.py to provide both a serialiser field and viewset. It then adds the audit log serialiser field to the RetrieveEYBLeadSerializer. The response will now include a field similar to the following:

    "audit_log": [
        {
            "id": 622,
            "user": null,
            "timestamp": "2025-02-18T07:00:00.123456Z",
            "comment": "",
            "changes": {
                "address_1": [
                    "Old address 1",
                    "New address 1"
                ]
            }
        }
    ]

Checklist

• Has this branch been rebased on top of the current main branch?

  Explanation

  The branch should not be stale or have conflicts at the time reviews are requested.

• Is the CircleCI build passing?

General points

Other things to check

• Make sure fixtures/test_data.yaml is maintained when updating models
• Consider the admin site when making changes to models
• Use select-/prefetch-related field lists in views and search apps, and update them when fields are added
• Make sure the README is updated e.g. when adding new environment variables

See docs/CONTRIBUTING.md for more guidelines.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.66%. Comparing base (4c93e3c) to head (b55e2b3).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5965   +/-   ##
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  Coverage   96.65%   96.66%           
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  Files        1079     1079           
  Lines       25353    25383   +30     
  Branches     1673     1675    +2     
=======================================
+ Hits        24505    24536   +31     
  Misses        691      691           
+ Partials      157      156    -1     

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