Implement Server-Sent Events(SSE) , Cross-Origin-Resource-Sharing(CORS)

It would be easy to implement web-client for KSQL-Server if SSE and CORS are implemented.
SSE provides better abstraction to represent streaming structured data (JSON) in the browser or server-side ([WebClient](https://docs.spring.io/spring-framework/docs/5.0.0.M1/spring-framework-reference/html/web-reactive.html)).
CORS helps to deploy KSQL-Server and Web-Clients on separate hosts.

The issue names SSE and CORS but no files or tests. Start by locating the KSQL-Server HTTP entry point and reviewing the linked Spring WebFlux reference; define the browser-facing streaming and cross-origin behavior, then verify both from a separate web client.

Written by the indexing model from the issue text.