feat: Add metadata fields to core entities (API)

[novakzaballa]

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• [x] I have run pre-commit to check linting
• [x] I have added information to docs/ if required so people know about the feature!
• [x] I have filled in the "Changes" section below?
• [x] I have filled in the "How did you test this code" section below?
• [x] I have used a Conventional Commit title for this Pull Request

Changes

FE changes:

• https://github.com/Flagsmith/flagsmith/pull/3212

BE changes:

• Support metadata for segments, and Flags
• Support required metadata model fields for segments, and Flags

How did you test this code?

Tests added

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 98.06763% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 96.00%. Comparing base (92e3e9f) to head (100f8eb).

Files	Patch %	Lines
api/segments/serializers.py	91.66%	2 Missing :warning:
api/features/serializers.py	94.11%	1 Missing :warning:
api/metadata/views.py	87.50%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3315      +/-   ##
==========================================
+ Coverage   95.99%   96.00%   +0.01%     
==========================================
  Files        1135     1135              
  Lines       36185    36361     +176     
==========================================
+ Hits        34735    34909     +174     
- Misses       1450     1452       +2     

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[novakzaballa]

@gagantrivedi I'm not sure what you're referring to here: https://github.com/Flagsmith/flagsmith/pull/3315#discussion_r1534062976