Add x-root-ancestor vendor extension boolean property to use inheritance w/o discriminator (see #6127).

[SergeyLyakhov]

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@bbdouglas @JFCote @sreeshas @jfiala @lukoyanov @cbornet @jeff9finger
cc: @wing328

Description of the PR

Add mechanism to support inheritance w/o discriminator.
Vendor extension property x-root-ancestor: [true|false] should be used to annotate the root model of hierarchy:

   Root:
    x-root-ancestor: true
    allOf:
      - type: object
        properties:
          id:
            type: string

  Intermediate:
    allOf:
      - $ref: '#/definitions/Root'
      - type: object
        properties:
          name:
            type: string

  Leaf:
    allOf:
      - $ref: '#/definitions/Intermediate'
      - type: object
        properties:
          description:
            type: string

This PR is related to #7593, and reimplemented according to requirements explained in comments there.

[pnewhook]

Any chance of getting this merged? I'm interested in #7060

[Irene93]

Is there any news about the release of this feature? Thank you

[TheOneValen]

This would really help with my Usecase, otherwise I can't use inheritance :-(

[rchalaszczyk]

Any chance for it? Needed for #7060

[vlauciani]

Is there any news about this PR?

[vlauciani]

About the inheritance problem without using the discriminator, I found this workaround:

• [JAVA] Inheritence without discriminator, is it possible ? #6127 (comment)