feat(schemas): Support partially gzip-compressed environment documents

[khvn26]

Contributes to Flagsmith/flagsmith#6394, https://github.com/Flagsmith/edge-api/issues/508

In this PR, we update the DynamoDB schemas with Compressed variants of the environment document.

A generic JsonGzipped type is introduced that, when used via a Pydantic type adapter, validates the inner type, then JSON-encodes and gzip-compresses it. On the consuming side, the type informs the data shape after decompression.

Gzip is used with mtime=0 and JSON encoding with sort_keys=True to support repeatable results. simplejson library is used so validated decimals are encoded natively.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.76%. Comparing base (775362d) to head (47fe4d1).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #157      +/-   ##
==========================================
+ Coverage   95.67%   95.76%   +0.08%     
==========================================
  Files          85       86       +1     
  Lines        2891     2949      +58     
==========================================
+ Hits         2766     2824      +58     
  Misses        125      125              

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[emyller]

Approved with non-blocking comments.