Add type: "null" downcasting when in oneOf and anyOf for OpenAPI v3

Description

This PR adds downcasting of oneOf and anyOf for openapi v3.

When writing OpenAPI v3 it will remove schemas from oneOf and anyOf with { "type": "null" } and apply it to the parent as nullable: true. It also tries to find a common overlapping type which will also be applied to the parent schema as nullable: true is only doing something when type is specified.

It's between a bug-fix and a feature as the current situation downcasts { "type": "null" } to { "nullable": true } which according to some clarifications of the spec doesn't do anything and tools expect the nullable: true on the parent schema.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Other (please describe):

Related Issue(s)

• Downcast oneOf null to nullable with OpenAPI 3.0 #2629
• [OpenApi] How to use allOf dotnet/aspnetcore#64648

Changes Made

• Add detection and processing of compositions where there could be a type: null for OpenAPI v3.
• Apply either the type of the schema or the inferred type on the schema, a possible explicit type is not overwritten.
• When multiple possible types are detected it will omit the inferred type.

Testing

• Unit tests added/updated
• Integration tests added/updated
• Manual testing performed
• All existing tests pass

Checklist

• My code follows the code style of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Versions applicability

• My change applies to the version 1.X of the library, if so PR link:
• My change applies to the version 2.X of the library, if so PR link:
• My change applies to the version 3.X of the library, if so PR link:
• I have evaluated the applicability of my change against the other versions above.