Intern attribute type

[jcmosc]

This adds a OGUnownedGraphContextRef type which is actually a pointer to a OG::Graph instance.

The OGSubgraphGetGraph function, which is now unimplemented, needs to look up the OG::Graph::Context instance from a map of context IDs to Context instances stored on the OG::Graph instance.

The one inconsistency is OGGraphContextGetGraph which still takes a raw pointer to a OG::Graph::Context instance. Though this function is only called from AGSubgraphGetGraph and looks to be inlined.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 4.92%. Comparing base (65a976c) to head (5dd2d74).

Files with missing lines	Patch %	Lines
...rces/OpenGraph/Attribute/Attribute/Attribute.swift	0.00%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main    #173      +/-   ##
========================================
+ Coverage   4.89%   4.92%   +0.02%     
========================================
  Files         28      28              
  Lines        817     813       -4     
========================================
  Hits          40      40              
+ Misses       777     773       -4     

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