[filebeat][streaming] - Standard OAuth2 implementation for websockets.

[ShourieG]

Describe the enhancement: Current OAuth2 implementation uses just a bearer token to authenticate and establish the connection. Due to the nature of websocket connections these tokens are usually long-lived and generally a token refresh is not required. But having traditional OAuth2 is oftentimes necessary for better security purposes.

Describe a specific use case for the enhancement or feature: The initial handshake in a websocket connection is done over HTTP/HTTPS. This is the point where we need to implement OAuth2, after which, if successful a standard websocket connection will be established. Since we are using gorilla websockets library, this will need to be performed before we upgrade the connection to websocket.

ER: https://github.com/elastic/enhancements/issues/23116

[elasticmachine]

Pinging @elastic/security-service-integrations (Team:Security-Service Integrations)