api: add RESTful API for controlling the sampling profiler

Starting:

   curl -X POST http://osv:8000/trace/sampler?freq=1000

Stopping:

   curl -X POST http://osv:8000/trace/sampler?freq=0

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>