Constraints: expose struct fields as profiles

From https://jira.jpl.nasa.gov/browse/AERIEQS-442:

E.g. we have an Activity with a 3 vector parameter that within the constraint system generates a Discrete parameter with a {x: number; y: number, z: number} schema. I want to grab the x value from that and check if it is less than something, but can’t do that while it’s a Discrete

The scope of this task would be to make is possible to refer to a numeric field of a struct discrete profile (whether a resource profile, or an activity parameter). Ideally, this would also handle deeper nesting.

Non-struct discrete profiles (e.g. lists) would be out of scope for this ticket.

Very notional syntax:
Where, /vector is a discrete resource, which is a struct with a numeric field "x".

Real.Resource("/vector", "x").lessThan(5)

These profiles would probably be limited to step functions - though we could give some thought to whether we could have a non-zero slope linear profile as a field of a struct. (this topic came up when we were walking through #546, I forget where we arrived).

Related to #757

Start by reviewing the constraint-system handling of discrete resource profiles and activity parameters, then read related issue #757 and the discussion referenced from #546. Define how numeric fields and deeper nested fields are selected, including whether field profiles are limited to step functions; done means struct fields can be referenced in constraints while list profiles remain out of scope.