Introduce `itemLabel` on `fields.relationship`

[tordans]

Right now, the relationship dropdown is displayed as a list of relation ids.

It would be great to be able to customize what is shown there in a way similar to what fields.array has with itemLabel.

Docs: https://keystatic.com/docs/fields/relationship

Example of what it looks like now:

Example: I can fake this today by building the relationship manually and use fields.select

(The width #1298 is a different issue here.)