Support x-tagGroups in lint command

[AlexVarchuk]

Is your feature request related to a problem? Please describe.
We have too many useful specification extensions. But openapi-cli doesn't support all of these.

Describe the solution you'd like
I think we should start supporting this from the lint command.

Describe alternatives you've considered
No. Because we should be consistent and already supported all custom extensions which describe in the documentation.

For example x-tagGroups:

x-tagGroups:  
  - name: test name
    tags: test_tag
    x-displayName: Test

[adamaltman]

This is good because it is one of the more popular ones. Others would be x-logo, x-codeSamples, and x-enumDescriptions.