Add HTTP header injection in HTTP Monitors

PageSequenceMonitor and HttpPostMonitor are able to do POST requests. It would be useful to enhance PSM to be able to set HTTP headers. This kind of monitor/heartbeat would allow us to integrate with on-call tools, e.g. OpsGenie, Pagerduty, Hot Potatoe and others.

Here some API examples from tools who support heartbeats:

• https://docs.opsgenie.com/docs/heartbeat-api
• https://docs.hotpotato.nz/api.html