feat: add x-explicitMappingOnly extension

[zeapo]

This replaces #774

When the mapping is partial, here the dog mapping was removed:

      discriminator:
        propertyName: petType
        mapping:
          cat: '#/components/schemas/Cat'
          kitten: '#/components/schemas/Cat'
          bee: '#/components/schemas/HoneyBee'

The type Dog will be included in addition to the mappings:

The idea is to have an exhaustive mapping, in that case we'd like to ignore non-mapping child classes. I've added a new extension x-limitToMapping of type boolean. When used, it excludes the class Dog in the following case:

      discriminator:
        propertyName: petType
        x-limitToMapping: true
        mapping:
          cat: '#/components/schemas/Cat'
          kitten: '#/components/schemas/Cat'
          bee: '#/components/schemas/HoneyBee'

will yield the following result:

[adamaltman]

Thanks for rewriting this clearly and referencing the original issue. 👍 👍

[RomanHotsiy]

Ooh! @zeapo sorry for not responding 🙈 . Tons of work here.

This PR is so much shorter 😍! Good job!

I'm not just fully sold on the name x-limitToMapping

@adamaltman do you have any other ideas? @zeapo maybe you have some alternative?

[zeapo]

Thanks :)

Maybe x-explicitMappingOnly? As you've named them explicit/implicit.

[RomanHotsiy]

Well, it is how they call it in the spec:

I like x-explicitMappingOnly more. Could you rename it and also add some short docs to https://github.com/Redocly/redoc/blob/master/docs/redoc-vendor-extensions.md

As soon as you're done I am ready to merge this 👍

[zeapo]

@RomanHotsiy , It's done. Let me know if anything is missing.

[adamaltman]

I think this is a good addition.

[RomanHotsiy]

@zeapo awesome! Thanks 🙇