Relation search widget

[cassiozen]

- Summary

Creates a "Relation" widget (search based).

Config example:

- {label: "Author", name: "author", widget: "relation", collection: "authors", searchFields: ["first_name", "last_name"], valueField: "title"}

- Description for the changelog

Search-based relationship widget

- A picture of a cute animal (not mandatory but encouraged)

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