feat: Add DynamoDB document schemas

[khvn26]

Contributes to #128.

In this PR, we add Python stdlib-based DynamoDB document schemas for Flagsmith's Edge API data model.

The goals are to:

• Preserve a single source of truth for Edge API's data model.
• Thoroughly document it, and uphold high documentation standards in the future.
• Decouple the schema from its validation and serialisation code so the data can be consumed by typed Python code without having to rely on Pydantic dependency.
• At the same time, make it usable for serialisation tools such as Pydantic and preserve compatibility with existing validation and serialisation behaviour.

Initially, I was hesitant to add validation code to this package. I'm considering to add it now, however, in a subsequent PR, so it can be consumed in environments where Pydantic is still possible to use.

I've added a bunch of TODOs that I expect to resolve prior to merging the current PR, and highlighted them by starting relevant review threads.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.31933% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.32%. Comparing base (51c529b) to head (43a7efb).

Files with missing lines	Patch %	Lines
src/common/test_tools/plugin.py	33.33%	4 Missing ⚠️

❌ Your patch status has failed because the patch coverage (98.31%) is below the target coverage (100.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@                Coverage Diff                 @@
##           ci/uv-renovate     #137      +/-   ##
==================================================
+ Coverage           94.72%   95.32%   +0.60%     
==================================================
  Files                  76       79       +3     
  Lines                2444     2673     +229     
==================================================
+ Hits                 2315     2548     +233     
+ Misses                129      125       -4     

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[Zaimwa9]

Approving with a NIT wording, i leave it up to you. Otherwise I consider @gagantrivedi' comments to have been resolved

Gagan is out today; all of his threads were addressed and double-checked by a peer.