Add Stream-based Control Flow for Observation Emitter

This commit introduces tokio streams to control how often a third-party
adapter is called to fetch data from a data source. The adapter is called
on an interval using a timer, and the results are chunked before being
returned as a stream.

It adds a simple stream-based pipeline to gather observations
as they're emitted by a local cache. It plumbs together a futures API
to make event handling nature and easy.

chore: Add stream-based control flow for third-party adapters.