feat: Support JSON Serialize for Value

[WeihanLi]

This PR

• add ValueJsonConverter to support STJ JSON Serialize

Related Issues

Fixes #517

Notes

• Add ValueJsonConverter to support STJ JSON Serialize, keep it internal for now, think we could make it public when necessary

Follow-up Tasks

• Do we need to add a ToJsonString() method likes LD SDK?
• Do we need to add a method to convert Value to a strong-typed T?

How to test

refer to the unit test cases

[codecov]

Codecov Report

Attention: Patch coverage is 96.10390% with 3 lines in your changes missing coverage. Please review.

Project coverage is 88.18%. Comparing base (a0ae014) to head (909377b).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/OpenFeature/Model/ValueJsonConverter.cs	96.10%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #529      +/-   ##
==========================================
+ Coverage   87.88%   88.18%   +0.30%     
==========================================
  Files          48       49       +1     
  Lines        1972     2049      +77     
  Branches      222      239      +17     
==========================================
+ Hits         1733     1807      +74     
  Misses        191      191              
- Partials       48       51       +3     

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[askpt]

LGTM! Thanks for your contribution.