mrc-5092 Proof of concept: support basic auth for Montagu

[EmmaLRussell]

See the corresponding montagu-proxy PR for full explanation - we will support Montagu by supporting basic auth. This partial implement just includes a hard-coded test user.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 91.92%. Comparing base (7991814) to head (ada3725).
Report is 12 commits behind head on main.

❗ Current head ada3725 differs from pull request most recent head 93b158c. Consider uploading reports for the commit 93b158c to get more accurate results

Files	Patch %	Lines
app/src/app/components/login/UserAuthForm.tsx	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #53      +/-   ##
==========================================
- Coverage   92.09%   91.92%   -0.17%     
==========================================
  Files          64       64              
  Lines         506      508       +2     
  Branches      128      130       +2     
==========================================
+ Hits          466      467       +1     
- Misses         38       39       +1     
  Partials        2        2              

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