Return content type in header response for /ksql endpoint

[vpapavas]

Describe the bug
We added the content type in the response of the \query endpoint (#9282) but did not apply the change to the other endpoints. We need to apply it to all endpoints so that users can do the following:

If one wants to disable HSTS headers, they would try

response.http.headers.config=add Strict-Transport-Security: max-age=63072000

The expectation is that we would see strict-transport-security: max-age=63072000 in the response HTTP headers, as an example for broker:

curl -k -i https://localhost:8090/kafka/v3/clusters
HTTP/2 200
date: Tue, 20 Feb 2024 19:08:03 GMT
strict-transport-security: max-age=63072000
content-type: application/json
vary: Accept-Encoding, User-Agent

I do not see it for ksqlDB:

curl -k -i https://localhost:8088/info
HTTP/2 200
content-type: application/json
content-length: 132

{"KsqlServerInfo":{"version":"7.4.1","kafkaClusterId":"oIXR8Hc-Qxq4_-rA7XN1-w","ksqlServiceId":"default_","serverStatus":"RUNNING"}}