[BUG] CAT endpoints response bodies' content-types are application/json

[nhtruong]

While the vast majority of OpenSearch endpoints' response bodies are in JSON format, CAT endpoints return tabular data as plaintext instead. The spec currently uses application/json for these endpoints, which is incorrect.

[Xtansia]

They can and will return JSON if you pass ?format=json which is what a number of the language clients are hardcoded to do

[dblock]

Note that https://localhost:9200/_cat?format=json doesn't return JSON. AFAIK there's no way in OpenAPI to say parameter X -> this kind of output, or is there?

[dblock]

This was fixed in #360 and #357.