feat: externalValue resolver

Content & configuration

https://editor.swagger.io/?url=https://gist.githubusercontent.com/mathis-m/262703ffd14ac670366e603d0143943f/raw/4d4d7f3f5a5ba27c9880344976d7297e600588ce/tm8jDkQ5Cq.txt
Swagger/OpenAPI definition:

openapi: 3.0.0
info:
 title: asd
 version: "123"
paths:
 /:
   post:
     parameters:
       - in: header
         name: limit
         schema:
           type: integer
           maximum: 50
         examples:
           max:
             summary: A sample limit value  # Optional description
             externalValue: 'https://gist.githubusercontent.com/mathis-m/ad32902aa510d7e69b4b8d3b02e284a6/raw/613955421088a491231b1c6c7766f0f95a5c5824/vNwATb2zAG.txt'
     requestBody:
       content:
         application/json:
           schema:
             {}
           examples:
             jsonObject:
               summary: A sample object
               externalValue: 'https://gist.githubusercontent.com/mathis-m/e97d480acd5e13455beeba37947d2d8b/raw/9e26dfeeb6e641a33dae4961196235bdb965b21b/z1xx2GHQEY.txt'
         image/png:
           schema:
             type: string
             format: binary
           examples:
             KittenImage:
               externalValue: "https://placekitten.com/200/300"
     responses:
       "200":
         description: OK!
         content:
           application/json:
             schema:
               {}
             examples:
               jsonObject:
                 summary: A sample object
                 externalValue: 'https://gist.githubusercontent.com/mathis-m/e97d480acd5e13455beeba37947d2d8b/raw/9e26dfeeb6e641a33dae4961196235bdb965b21b/z1xx2GHQEY.txt'

Is your feature request related to a problem?

I think #5433 at swagger-ui is only partially a swagger-ui issue!
I have tried to integrate externalValue examples (from params, requestBody, response) into the sample generation process.
I realized that this value is not resolved, I think swagger-js should resolve this just like it resolves refs.
Originally posted by @mathis-m in https://github.com/swagger-api/swagger-ui/issues/5433#issuecomment-797645483
.

Describe the solution you'd like

I think swagger-js should resolve this just like it resolves refs.

Describe alternatives you've considered

Else ways I will implement some resolver that will fetch the raw value hosted behind the externalValue url within swagger-ui

Additional context

#5433 at swagger-ui

I have started implementing this and will create a PR for this if it is appreciated in a few days.