feat: add the ability to link existing documents in the join field

[illright]

What?

This PR adds a new button to the Join field UI to take an existing document from the joined collection and link it to this joining document instead.

Why?

This is a custom functionality that I needed as part of my own usage of Payload. I found the integration to be a bit difficult — I had to modify the label component and extend it in width up to the Add new button to give an impression of a second action. Not to mention that after sending a PATCH request to do the linking, I had to call window.location.reload() because I couldn't come up with a better way to invalidate the table from within a custom component.

Since the new functionality was mostly using existing Payload components, I figured I'd try to contribute it back in case you decide that it's useful.

How?

This uses the ListDrawer element to display a slide-out panel to browse existing documents that can be linked to the current document.

Note

This PR is still not finished — after selection, the document isn't actually being linked. I wanted to get your input on whether this contribution is welcome, and if so, what would be the best way to implement the linking? The only thing I can think of is a fetch call, but perhaps there's a better way to do updates from inside Payload client components