[FEATURE] Add support for flags (parameters without value)

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Is your feature request related to a problem?

The _cat API takes flags in the form of ?help without a value. In #360 I added support for boolean parameters such as ?help=true. OpenSearch also accepts it as a flag, ?help. A null value or an empty string don't work.

  - synopsis: Cat displaying all available headers (help).
    method: GET
    path: /_cat/health
    parameters:
      help: null
    response:
      status: 200
      content_type: text/plain

What solution would you like?

Not sure.

Do you have any additional context?

This was discussed in OAI/OpenAPI-Specification#1782.