feat: `HealthManager` should provide an events-based API

[danisharora099]

Problem

HealthManager's role is to aggregate and provide data around individual protocols' health status (number of peers connected for a protocol) in the form of async methods: getProtocolHealth()/getOverallHealth().

On consumer-side, these methods allow access to information, but live updates are not available. Workaround has been to call these methods in intervals of time. This creates a delay and adds friction to promptly be able to respond to the node's state of connections.

Solution

Provide an events-based API for changes to the protocols' health statuses

cc @vpavlin

[weboko]

Let's build a simple API around ConnectionManager and expose amount of peers per protocol.
Event based API can be build on top of libp2p events similarly to connection state on ConnectionManager.