Expose External Events to Procedural Scheduling Goals

[dandelany]

Background

V1 of Procedural Scheduling was released in Aerie v2.19. In the meantime, the GSFC team has been working on implementing the External Events feature & has submitted PRs (under review) for that. Currently they are completely separate features and Procedural Scheduling goals don't know about external events.

Description

In future, procedural scheduling goals should be able to access external events attached to a plan, allowing users to eg. schedule certain activities every time a particular external event occurs. This should be pretty similar to the existing Java/Kotlin interface used to get activity directives from a plan.

@JoelCourtney has met with @JosephVolosin & @pranav-super about this to define a basic interface spec, and is already working on implementing it in a branch - PR coming soon (aiming for release in v2.21)

Dependencies

This work will be based on the feature/external-event branch, so the External Events PRs need to be merged first. Will also depend on the code in #1560