refactor: allow any token retriever in the rest client

[jacob720]

See https://confluence.diamond.ac.uk/pages/viewpage.action?pageId=398853592&spaceKey=DCT&title=04-06-2026
Currently the rest client requires a session manager to handle retrieving auth tokens. I've made it more flexible so that it now accepts anything with a get_valid_access_token method, so a UDC client can use the same rest client.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.61%. Comparing base (ab2ebc6) to head (8e1d330).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1553   +/-   ##
=======================================
  Coverage   95.61%   95.61%           
=======================================
  Files          43       43           
  Lines        3214     3215    +1     
=======================================
+ Hits         3073     3074    +1     
  Misses        141      141           

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