Support returning existential types in functions

[Matejkob]

Add support for such a functions:

@Spyable protocol Foo {
  func get() -> any Codable
}

[codecov]

Codecov Report

Attention: Patch coverage is 86.66667% with 4 lines in your changes missing coverage. Please review.

Project coverage is 97.92%. Comparing base (16f7832) to head (fbf2d7f).

Files	Patch %	Lines
...es/SpyableMacro/Factories/ReturnValueFactory.swift	86.66%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #97      +/-   ##
==========================================
- Coverage   98.39%   97.92%   -0.48%     
==========================================
  Files          18       18              
  Lines         749      772      +23     
==========================================
+ Hits          737      756      +19     
- Misses         12       16       +4     

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