REST configuration listeners

Add configuration event listeners to the REST endpoint.
These should use HTTP Server-Sent-Events
https://html.spec.whatwg.org/multipage/server-sent-events.html

Configuration listeners:

GET /v2/server/config?action=listen[&includeCurrentState=true|false]

This will result in a response with content-type text/event-stream followed by a stream of configuration events.
Each event follows the following structure:

event: [cache|template|schema|task]
data: [json representantion of event]

the data field will contain a representation of the event:

• [cache|template|counter]-created: the full configuration of the entity
• [cache|template|counter]-removed: an empty string
• [schema]-created: the protobuf
• [schema]-removed: an empty string

This listener can be used by the operator to properly implement reconciliation of CRs when the entities they refer to are created/modified via other means (API, CLI, Console).