[Epic] Two-way communications protocol

[andrewazores]

The agent currently exposes only a readonly HTTP API that the Cryostat server can use to query basic information such as a list of active recording descriptors, the JFR event types and templates available, and MBean metrics data.

The agent should also implement mutation requests for actions such as dynamically starting Flight Recordings, including with custom event templates supplied by the server with the request.

The readonly API should remain implemented and available. We should explore whether the mutable (write/update) API endpoints are always available, or if they are gated behind an additional opt-in property that requires the user to explicitly enable them.

The goal after this project is completed is that the Cryostat backend's client should be able to make all of its requests to an HTTP-registered Agent instance equally as it would for a JMX target.

• [Story] Separate TLS keystore/truststore from host application #138
• [Story] Dynamically start Flight Recordings #162
• [Task] Download recordings over HTTP #178

[andrewazores]

Related: cryostatio/cryostat-web#944

[andrewazores]

#138 should still be done at some point.