fix!: Add support for missing attributes in ProjectsV2 API

[tmelliottjr]

This PR addresses a few gaps found between the current implementation and GitHub's ProjectsV2 OpenAPI spec:

• Introduced new ProjectV2ItemContent and ProjectV2ItemWithContent structs, along with a custom UnmarshalJSON implementation to inspect the content_type returned by the API and automatically unmarshall the item's content (Issue|PullRequest|DraftIssue) into the correct struct.
• Added accessor methods for ProjectV2, ProjectV2DraftIssue, ProjectV2ItemContent, ProjectV2ItemWithContent, and ProjectV2StatusUpdate structs.
• Changed the ContentType field on ProjectV2Item from *string to a dedicated *ProjectV2ItemContentType type for improved type safety.
• Added IsTemplate and LatestStatusUpdate to the existing ProjectV2 struct.

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.47%. Comparing base (e1238d0) to head (f712cfb).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
github/projects.go	92.85%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3888      +/-   ##
==========================================
- Coverage   92.48%   92.47%   -0.01%     
==========================================
  Files         200      200              
  Lines       14564    14594      +30     
==========================================
+ Hits        13469    13496      +27     
- Misses        895      897       +2     
- Partials      200      201       +1     

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[gmlewis]

I'm wondering if we need a brand new "...WithContent" type here?
Since this PR is breaking the API anyway, can't you just do all the unmarshaling into the original "ProjectV2Item" struct?

It seems odd to me to continue to have both a "ProjectV2Item" and a "ProjectV2ItemWithContent" (and the extra maintenance burden that causes) if we can avoid it.

Thoughts?