feat: Remove restrictions for union/either types

[Natoandro]

In the previous versions, we restricted the variant types allowed in union/either to be all in the same category (after flattening multi-level unions):

• Category 1 - GraphQL leaf types: scalar type, or array of scalar type, which require no selection set on GraphQL.
• Category 2 - GraphQL non-leaf types: object type or array of object type, which require a selection set on GraphQL (aka selectable types in the codebase).

Those restrictions can be lifted, and the selection field of an union-type field will have inline fragments with type conditions for each Category 2 variant. No type condition is required for Category 1 types, the selection sets are not relevant.

The case that is not handled by this PR is when one of the variants is an array of union type.

Migration notes

N/A

• The change comes with new or modified tests
• Hard-to-understand functions have explanatory comments
• End-user documentation is updated to reflect the change

[linear]

MET-402 Remove restrictions for union/either

[codecov]

Codecov Report

Attention: Patch coverage is 84.26230% with 48 lines in your changes missing coverage. Please review.

Project coverage is 73.49%. Comparing base (3ed6e1e) to head (d505cdb).

Files	Patch %	Lines
typegate/src/engine/typecheck/result.ts	74.39%	21 Missing ⚠️
typegate/src/engine/planner/mod.ts	81.69%	12 Missing and 1 partial ⚠️
typegate/src/typegraph/utils.ts	87.87%	12 Missing ⚠️
typegate/src/typegraph/mod.ts	92.59%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #761      +/-   ##
==========================================
+ Coverage   73.38%   73.49%   +0.10%     
==========================================
  Files         117      117              
  Lines       14145    14195      +50     
  Branches     1409     1421      +12     
==========================================
+ Hits        10381    10432      +51     
  Misses       3738     3738              
+ Partials       26       25       -1     

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