Investigate control plane interface needed to interoperate with the Datadog Agent.

[tobz]

Context

Currently, Agent Data Plane interacts with the Datadog Agent in a very limited way, scoped narrowly to use the core Agent as a remote tagger service, which supports origin enrichment.

However, as we aim to integrate more deeply with the Datadog Agent, we will quickly need something closer to a true control plane. Here's a non-exhaustive list of things we expect to need the core Agent and ADP to be able to communicate with each other about:

• configuration reloading (e.g. pulling an updated API key from the core Agent and updating any Datadog-based destinations)
• status reporting (being able to query ADP for its status when an operator runs agent status)
• flare creation (being able to trigger ADP to generate and collect any relevant information when creating a flare)

[tobz]

Part of this -- support for status and flare creation -- is likely to be tackled through the addition of a new shared component on the Agent side: DataDog/datadog-agent#30543