Add support for $ref in requestBody of endpoint

[Eruzo]

This pull request adds support for the $ref keyword in request bodies. Given the following oas endpoint:

/end-point-uri:
    post:
      tags:
        - SomeTag
      summary: Search for something with a request body
      description: |
       Search something with a request body that is defined as a reference
      operationId: PostSearchSomething
      parameters:
        - $ref: '#/components/parameters/CorrelationId'
        - $ref: '#/components/parameters/ReferenceDate'
      requestBody:
        $ref: '#/components/requestBodies/PostSearchSomethingRequest'

The generator would generate the following function PostSearchSomethingRequestAsync(object body) with the supplied change it will generate PostSearchSomethingRequestAsync(PostSearchSomethingRequest body).

Changes to [src/NSwag.Core/OpenApiDocument.Serialization.cs] and [src/NSwag.Core/OpenApiComponents.cs] are probably not needed.

Fixes #4093 as well I think