Generate sample bulk add trainee CSVs

[stevehook]

Context

This PR just extends (and refactors) a rake task that we built to generate sample data for the bulk CSV trainee upload feature.

Changes proposed in this pull request

This PR adds options to:

• Include invalid values (to test error handling)
• Include missing required values (to test error handling)
• Include placement data
• Include degree data

It also extracts all the logic to a SampleTraineeDataGenerator service class

Guidance to review

Do the generated files work (or fail) as expected?

Important business

• Does this PR introduce any PII fields that need to be overwritten or deleted in db/scripts/sanitise.sql?
• Does this PR change the database schema? If so, have you updated the config/analytics.yml file and considered whether you need to send 'import_entity' events?
• Do we need to send any updates to DQT as part of the work in this PR?
• Does this PR need an ADR?

NB: Please notify the #twd_data_insights team and ask for a review if new fields are being added to analytics.yml

[codecov]

Codecov Report

Attention: Patch coverage is 94.11765% with 3 lines in your changes missing coverage. Please review.

Project coverage is 96.09%. Comparing base (d5bfb26) to head (e291751).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...vices/bulk_update/sample_trainee_data_generator.rb	94.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4912      +/-   ##
==========================================
- Coverage   96.10%   96.09%   -0.01%     
==========================================
  Files         743      744       +1     
  Lines       16050    16101      +51     
==========================================
+ Hits        15425    15473      +48     
- Misses        625      628       +3     

Flag	Coverage Δ
unittests	96.09% <94.11%> (-0.01%)	⬇️

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[sonarqubecloud]

Quality Gate passed

Issues
4 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud