Fixing get_file_signals for custom types

[dtulga]

This fixes get_file_signals for proper custom type handling, removing the workaround previously added due to lack of custom type serialization / deserialization. This also adds a test to ensure these types are processed correctly, so that this can be caught earlier than Studio end-to-end tests.

This has been tested with my local Studio install to work with both old (no custom types) datasets and new datasets (with custom types) with both custom type display and image preview working. This is a followup to #348 and is part of https://github.com/iterative/studio/issues/10417 and https://github.com/iterative/studio/issues/10547

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.71%. Comparing base (477d7d5) to head (d9e8ada).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #371      +/-   ##
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- Coverage   86.72%   86.71%   -0.01%     
==========================================
  Files          91       91              
  Lines       10092    10085       -7     
  Branches     2051     2049       -2     
==========================================
- Hits         8752     8745       -7     
  Misses        985      985              
  Partials      355      355              

Flag	Coverage Δ
datachain	86.64% <100.00%> (-0.01%)	⬇️

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