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

[dblock]

Is your feature request related to a problem?

The _cat API takes flags in the form of ?help without a value. In https://github.com/opensearch-project/opensearch-api-specification/pull/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 https://github.com/OAI/OpenAPI-Specification/issues/1782.