Collection of subschema improvements and some general features

[perher]

This is a large collection of fixes that mostly improves on the subschemas (allOf/anyOf) support added in #99.

Keeping this as one large MR for now to reflect the larger work being done in my fork as this project seems less active nowadays.

New features:

• alias single anyOf/allOf types (Collapse anyOf with only one option? #296)
• support allOf/anyOf refs across different schemas
• const validation (Support for const property validation #472)
• omitzero tag support (Support for omitzero tag added in Go 1.24 #471)
• infer missing object type in some cases

Fixes:

• support direct properties for allOf/anyOf types (v0.18.0 combination of anyof and required loses all information about object #431, also replaces fix: merge allOf, properties, and additionalProperties #464, fix: support allOf and anyOf with with direct properties #465)
• missing anyOf validators when using refs (v0.18.0 regression: anyOf with null variant results in incorrect codegen #427)
• compile error with anyOf validators (v0.18.0 regression: anyOf with null variant results in incorrect codegen #427)
• support external refs with allOf/anyOf
• avoid method duplicates in codegen
• add newline after alias types
• only process same schema once

Breaking changes:

• anyOf could mark properties as required even if they were only required in one of the subschemas

Includes PRs

• fix: support allOf and anyOf with with direct properties #465

[codecov]

Codecov Report

❌ Patch coverage is 22.33918% with 664 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@4b5de3c). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
tests/data/core/anyOf/anyOfMultipleRequired.go	0.00%	124 Missing ⚠️
tests/data/core/allOf/allOfMultipleRequired.go	0.00%	118 Missing ⚠️
tests/data/core/anyOf/anyOfWithDirectProperties.go	0.00%	86 Missing ⚠️
tests/data/core/allOf/allOfWithAnyOf.go	0.00%	80 Missing ⚠️
tests/data/core/anyOf/anyOf.ref.go	0.00%	80 Missing ⚠️
tests/data/core/allOf/allOfWithDirectProperties.go	0.00%	76 Missing ⚠️
tests/data/core/allOf/allOfNestedRefs.go	0.00%	60 Missing ⚠️
pkg/generator/schema_generator.go	77.77%	14 Missing and 8 partials ⚠️
pkg/schemas/model.go	79.16%	10 Missing and 5 partials ⚠️
pkg/schemas/types.go	50.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #473   +/-   ##
=======================================
  Coverage        ?   39.83%           
=======================================
  Files           ?       68           
  Lines           ?     7420           
  Branches        ?        0           
=======================================
  Hits            ?     2956           
  Misses          ?     4183           
  Partials        ?      281           

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[perher]

Above are tagged as v0.21.0-beta.0 in https://github.com/perher/go-jsonschema