Support dropdown for discriminator

[YousefHaggy]

Fixes #2438 and adds a dropdown to select between schemas that extend a base schema via allOf with discriminator

Sample spec:
spec.json

Description

• When parsing the spec, build a map of parent to child schemas
• In ModelWrapper, if the current schema has child schemas, render a drop down to select from those schemas
• Supports both explicit and implicit mappings

Motivation and Context

OpenAPI spec supports inheritance via allOf and discriminator, but currently in Swagger UI when you reference a schema that is extended with allOf, there's no indication of this inheritance and valid child schemas a user can input. Provides a poor user experience for specs that rely on this kind of structure

This works in other tools like Redoc: https://github.com/Rebilly/ReDoc/blob/master/docs/images/discriminator-demo.gif

Fixes #2438. Solves a very common allOf usecase, some examples in the OAS 3.1.1 official spec

How Has This Been Tested?

I've manually tested this change using the attached sample spec. I tested for both 3.0 and 3.1. I tested a spec with no explicit mappings, all explicit mappings, and a mix

spec.json

Ran all existing unit tests successfully. Tried to run npm run e2e as the README.md says, but no script e2e exists

Screenshots (if appropriate):

Checklist

My PR contains...

• No code changes (src/ is unmodified: changes to documentation, CI, metadata, etc.)
• Dependency changes (any modification to dependencies in package.json)
• Bug fixes (non-breaking change which fixes an issue)
• Improvements (misc. changes to existing features)
• Features (non-breaking change which adds functionality)

My changes...

• are breaking changes to a public API (config options, System API, major UI change, etc).
• are breaking changes to a private API (Redux, component props, utility functions, etc.).
• are breaking changes to a developer API (npm script behavior changes, new dev system dependencies, etc).
• are not breaking changes.

Documentation

• My changes do not require a change to the project documentation.
• My changes require a change to the project documentation.
• If yes to above: I have updated the documentation accordingly.

Automated tests

• My changes can not or do not need to be tested.
• My changes can and should be tested by unit and/or integration tests.
• If yes to above: I have added tests to cover my changes.
• If yes to above: I have taken care to cover edge cases in my tests.
• All new and existing tests passed.

[YousefHaggy]

Is there a better way to get specAction down to model-wrapper.jsx than plumbing it down everywhere like this? I couldn't find a context provider with specActions in the hierarchy

[YousefHaggy]

for OAS 3.0.X, the model is inline with the dropdown if we don't specify this

[YousefHaggy]

@char0n Could I get you or another maintainers review on this? Thanks!