(improvement, yaml): Update default OAuth configuration

[amckinney]

This builds upon the changes introduced in #3563, and adds the following:

• Update YAML configuration to expose clientId, clientSecret and scopes request properties.
• Add default configuration to simplify the expected use case.
• Refactor fern check to include more validation around the added configuration.

With this, the simplest OAuth configuration available to the user is the following:

name: api
imports:
  auth: auth.yml

auth: OAuthScheme
auth-schemes:
  OAuthScheme:
    scheme: oauth
    type: client-credentials
    get-token:
      endpoint: auth.getToken