add cors http headers to demo agent cfg

[robot-ranger]

Added cors http headers to make demo agent accessible from webapps for testing and demonstration.

I added the following to demo/compose/agent/agent.cfg:

HttpHeaders {
	Access-Control-Allow-Origin = *
	Access-Control-Allow-Methods = GET
	Access-Control-Allow-Headers = Accept
}

Allows all origins, but limits to GET requests. Im not a security professional so tell

closes #506