feat(flowcontrol): add foundational types

Introduces the foundational packages for the new Flow Controller
component.

This change includes:
- The top-level README outlining the motivation, high-level
  architecture, and component pillars.
- The `types` package, which defines the core data contracts, request
  lifecycle interfaces, error-handling vocabulary, and final outcome
  enums for the entire module.

This foundational PR establishes the core concepts and data models upon
which the rest of the Flow Controller implementation will be built.

Author: LukeAVanDrie