Following a similar model that we've previously used in other parts of
Terraform to expose UI-oriented progress events, here we define
stackruntime.Hooks as a collection of optional callbacks a caller can
use to subscribe to events, and the supporting utility code to help us
concisely and correctly call those hook functions from the main evaluation
code without adding too much extra noise to the code.
Although the primary focus of this API is driving realtime UI updates,
the API design also aims to accommodate distributed tracing use-cases by
allowing callers to propagate arbitrary values between related event
notifications and to inject additional values into the context.Context
used for downstream operations that might not be under the direct
supervision of stackruntime/stackeval.
