[rest] server-sent events

[OmegaRelay]

Server Sent Events in REST API server

Is your feature request related to a problem? Please describe. A clear and concise description of what the problem is.

At my company we currently use the http REST API to control the otbr instance with our own application.
The issue is we currently need to poll the /node/state endpoint to be updated of the connection status.

Describe the solution you'd like A clear and concise description of what you want to happen.

Server-sent event implementation defined in RFC-8895 for http clients to recieve push updates by starting a session with a GET request on a stream endpoint e.g /node/state/stream.

Describe alternatives you've considered A clear and concise description of any alternative solutions or features you've considered.

We've considered using the D-BUS api but opted for the REST api for the following reasons:

• We do not want our application running with the escalated privileges that is required to connect to the system daemon.
• We prefer a http && json data implementation due to our current knowledge base
• Application portablility is important to us

Additional context Add any other context or screenshots about the feature request here.