Add API endpoints to find and return client-side OTF directories

[tieneupin]

• The backend server will send a GET request to the instrument server requesting for OTF directories
• The instrument server will load the machine config and look for top-level folders under the configured gain_reference_directory field

Actual diff is about +90/-1, with the rest of the diff being due to the addition and modification of tests.

NOTE: This PR is tied to DiamondLightSource/murfey-frontend#81 and will have to be deployed along side the version that contains it.

[codecov]

Codecov Report

❌ Patch coverage is 92.59259% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 54.72%. Comparing base (4b78c30) to head (2206baf).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #876      +/-   ##
==========================================
+ Coverage   54.60%   54.72%   +0.12%     
==========================================
  Files         101      101              
  Lines       11311    11337      +26     
  Branches     1503     1507       +4     
==========================================
+ Hits         6176     6204      +28     
+ Misses       4808     4806       -2     
  Partials      327      327              

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[stephen-riggs]

Looks fine to me, with one comment about an optional path