Better handling of Inline schema

[wing328]

Better handling of Inline schema based on feedback from the users.

allOf inline schemas (children) are not created by default.

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[devhl-labs]

This PR does not resolve the following:

openapi: 3.0.1
components:
  schemas:
    Language:
      type: object
      properties:
        name:
          type: string
          readOnly: true
    Clan:
      type: object
      properties:
        chatLanguage:
          # nullable: true
          allOf:
          - $ref: '#/components/schemas/Language'

Note the two different language classes. This was not an issue in 6.0.0 beta 914b539 Also note that the CatAllOf pointed out in the issue did exist in 6.0.0 beta. The declawed property was placed in it's own AllOf class. The AllOf class would not be needed if the declawed property were just put into the Cat model. The mustache templates shouldn't even need changing. I think the issues here are related to #13709

[ahmednfwela]

looking at the dart samples in
https://github.com/OpenAPITools/openapi-generator/tree/inline-schema-improve3/samples/openapi3/client/petstore/dart-dio

I see that the inline model files still remain but their reference is removed here

what might be causing this ?

[wing328]

https://github.com/OpenAPITools/openapi-generator/blob/inline-schema-improve3/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/src/model/cat_all_of.dart is left over from previous code generation.

I can confirm manually removing https://github.com/OpenAPITools/openapi-generator/blob/inline-schema-improve3/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/src/model/cat_all_of.dart and regenerating dart samples will recreate this file, which is the expected behaviour as part of this change.

[wing328]

UPDATE: started a new PR (#15682) using a different approach

[wing328]

UPDAT: #15682 has been merged.