Add custom loader/deserializer, drop field_map from filedrop module

[arhamchopra]

This PR adds custom loaders and deserializer to the filedrop module.
In addition, it removes support for field_maps in the module since the custom loaders and deserializers can provide more flexibility.

[github-actions]

Test Results

369 tests  +5   369 ✅ +5   6m 16s ⏱️ + 2m 22s
  1 suites ±0     0 💤 ±0 
  1 files   ±0     0 ❌ ±0 

Results for commit dd552de. ± Comparison against base commit 6bf61c0.

♻️ This comment has been updated with latest results.

[codecov]

Codecov Report

❌ Patch coverage is 96.82540% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.15%. Comparing base (6bf61c0) to head (dd552de).
⚠️ Report is 25 commits behind head on main.

Files with missing lines	Patch %	Lines
csp_gateway/server/modules/filedrop/filedrop.py	78.94%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #109      +/-   ##
==========================================
+ Coverage   90.12%   90.15%   +0.02%     
==========================================
  Files         121      121              
  Lines       10091    10141      +50     
==========================================
+ Hits         9095     9143      +48     
- Misses        996      998       +2     

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