[Heartbeat/Synthetics] Network Discovery Functionality

**Describe the enhancement:**
I would like Heartbeat can be able to scan a network, determine hosts/ip, and report back hosts' ICMP status, up and down.
Think NMAP but built-in. It can be run on specified intervals just like how we currently create monitors. The current way we can "scan" is by manually inputting ICMP monitors. This has become a hassle and super tedious.

**Describe a specific use case for the enhancement or feature:**
Our team has used other competitor's monitoring tools and they offer network discovery. It's an important thing for being able to quickly determine if a host is pingable or not.

Start with Heartbeat/Synthetics and the existing ICMP monitor flow to understand how monitors are configured and scheduled. The issue names no files or tests, so first locate those entry points and clarify the network-scan scope. Done would mean scanning a specified network at intervals and reporting discovered hosts with ICMP up or down status.

Written by the indexing model from the issue text.