Attachment store for assistant2 (#11327)

This sets up a way for the user (or Zed) to _push_ context instead of
having the model retrieve it with a function. Our first use is the
contents of the current file.

<img width="399" alt="image"
src="https://github.com/zed-industries/zed/assets/836375/198429a5-82af-4b82-86f6-cb961f10de5c">

<img width="393" alt="image"
src="https://github.com/zed-industries/zed/assets/836375/cfb52444-723b-4fc1-bddc-57e1810c512b">

I heard the asst2 example was deleted in another branch so I deleted
that here too since we wanted the workspace access.

Release Notes:

- N/A

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Co-authored-by: Marshall <marshall@zed.dev>

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