Dynamic values in relation filters

Description

[???]We add dynamic values to relation dropdowns in relation filters in order for Creators to:

understand how current hierarchical filters work;
disable hierarchical filters when they are not needed;
- configure custom filters that are currently impossible: for example, using to-many relations (ex. filter Task's suggested assignees by who's working on the Project) — see the back-refs for use cases.

Dynamic values appear when a relation Field on the left can be compared:

either to the current Entity's Field
Product Area = This Bug → Product Area
Owner is any of This Project → Assignees
- ☝️ can be a Lookup Field as well (workaround for deep dynamic values)
or to the current Entity itself (think self-relations)
Parent Feature = This Feature

We don't support Nested dynamic values like This Entity → Relation Field → Field.

The current value is immediately visible in the dropdown
When a dynamic value is chosen, the current value appears in the tooltip on hover
- This Bug → Product Area — "Integrations"
- This Bug → Affected Product Areas — "Integrations • Entity View • API +3 more"
  Filter by to-many dynamic values using "any of"

When a dynamic value is empty (ex. This Bug → Product Area is not set), we treat it as null constant: Empty dynamic value.

Project ←→ App → Feature
Picking App for a Feature
Projects contains This Feature → Project
Task ← User (Owner) ←→ Roles
Task ←→ Roles
Picking Roles for a Task
Users contains This Task → Owner

Import/export via Space template should work
Delete Field => the filter rule is broken
If relation filters contain a dynamic value that can't be resolved (ex. there is no This Entity in Forms and automations), all relation filters are disabled with an explanation label.
- 🦄 in the future, we can ignore only the dynamic parts of the rule

See Migrate existing hierarchical filters to dynamic values for migration rules. We release dynamic values and migrate old filters at the same time.