Add linting rule to validate nullable enums

Currently, the OpenAPI spec has this to say about nullable enums:

Despite running our API spec through the linter with a non-conforming nullable enum, it passed! The end result was that a consumer downstream was having trouble integrating because they could not generate a client from the spec

I think it would be a great benefit to add a rule that explicitly checks:

IF `type` is `enum` AND `nullable` is `true`: ensure null appears in the list of values

The temporary solution I was considering is a custom rule, but this feels like something that should come out of the box as a built-in rule

Here is an example of something that should not pass linting, but does:

        myEnumType:
          type: string
          nullable: true
          enum:
            - "foo"
            - "bar"
            - "baz"           

And this is what it should be in order to conform:

        myEnumType:
          type: string
          nullable: true
          enum:
            - "foo"
            - "bar"
            - "baz"
            - null           

conversely, this should also fail:

        myEnumType:
          type: string
          nullable: false
          enum:
            - "foo"
            - "bar"
            - "baz"  
            - null         

Start by tracing the built-in linter rules and the custom-rule path for OpenAPI schemas. Add coverage for nullable enums with and without null, then confirm the non-conforming examples fail linting while conforming examples pass.