ObservableAttribute (#3925)
* ObservableAttribute proof-of-concept
* restructue sensors, add int impl, unit test
* add vector3 sensor, cleanup constructors
* add more types
* account for observables in barracuda checks
* iterators for observable fields/props
* stacking, fix obs size in prefab
* use DeclaredOnly to filter members
* ignore write-only properties
* fix error message
* docstrings
* agent enum (WIP)
* agent enum and unit tests
* fix comment
* cleanup TODO
* ignore by default, rename declaredOnly param, docstrings
* fix tests
* rename, cleanup, revert FoodCollector
* warning for write-only, no exception for invalid type
* move observableAttributeHandling to BehaviorParameters
* autoformatting
* changelog
* fix up sensor creation logic