Add host provider that pulls from remote Core Agent.

[tobz]

Context

Currently, we have a host provider -- AgentLikeHostProvider -- that is meant to emulate the Datadog Agent's hostname resolution behavior. This provider does work, but it has some notable shortcomings in support and changes to the Datadog Agent's hostname resolution logic are still happening on a regular cadence.

It would be good to add a new host provider that instead talks to the Datadog Agent to retrieve the hostname. The Core Agent's internal API already exposes the GetHostname RPC which exposes the same hostname that the Core Agent detects. This would allow us to rely more on the existing logic in the Datadog Agent instead of maintaining parallel implementations of this logic ourselves.