feat: Support PREVENT_EMAIL_PASSWORD in backend

[khvn26]

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• I have added information to docs/ if required so people know about the feature!
• I have filled in the "Changes" section below?
• I have filled in the "How did you test this code" section below?
• I have used a Conventional Commit title for this Pull Request

Changes

This adds a permission class to ensure that the /api/v1/auth/login endpoint can not be used when PREVENT_EMAIL_PASSWORD setting is enabled.

How did you test this code?

Enabled the setting locally and tried to access the /api/v1/auth/login endpoint.

[vercel]

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3 Skipped Deployments

Name	Status	Preview	Comments	Updated (UTC)
docs	⬜️ Ignored (Inspect)			Oct 23, 2024 2:57pm
flagsmith-frontend-preview	⬜️ Ignored (Inspect)			Oct 23, 2024 2:57pm
flagsmith-frontend-staging	⬜️ Ignored (Inspect)			Oct 23, 2024 2:57pm

[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-e2e:pr-4765	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api-test:pr-4765	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-frontend:pr-4765	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-api:pr-4765	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-4765	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-4765	Finished ✅	Results ✅

[github-actions]

Uffizzi Preview deployment-57558 was deleted.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.22%. Comparing base (6d2ab58) to head (9d662ae).
Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4765   +/-   ##
=======================================
  Coverage   97.21%   97.22%           
=======================================
  Files        1172     1172           
  Lines       40535    40537    +2     
=======================================
+ Hits        39408    39411    +3     
+ Misses       1127     1126    -1     

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