Allow custom field controls to be defined for "relation" type fields

[michaelbromley]

Is your feature request related to a problem? Please describe.
If it currently possible to define custom controls for any custom field apart from the "relation" type.

Describe the solution you'd like
Allow custom field controls to be defined for relation fields just like any other field.

Additional context
This is the part that needs to be extended:

vendure/packages/admin-ui/src/lib/core/src/shared/dynamic-form-inputs/relation-form-input/relation-form-input.component.html

Line 26 in 63e97c7

<ng-template ngSwitchDefault>

[michaelbromley]

On further investigation this actually is possible to do already using the standard means of specifying UI controls. However, it is slightly more complex to get working, so improvements can be made to documentation on this. Also add a working example to real-world-vendure, selecting a "featured review" relation to Product.