Check that 'source' field points to a zarr group in ImageLabel

[dstansby]

It looks like we still need to implement

The image-label dictionary MAY contain a source key whose value MUST be a JSON object containing information on the image the label is associated with. If included, it MAY include a key image whose value MUST be a string specifying the relative path to a Zarr image group. The default value is "../../" since most labels are stored under a subgroup named "labels/" (see above).

[d-v-b]

This is not very well specified. Seems like it's the intersection of an arbitrary JSON object and the specific JSON object {"image": <str>}. I'm guessing the path must resolve to an actual image? In pydantic terms, I think we have to accept any JSON input, but if that input contains an "image" field, then we have to check that it's a relative path that resolves to a zarr group that is an ome zarr image.

[dstansby]

Yep, that's how I interpret the text.

[dstansby]

Is there actually any way for us to check this, given it can contain a relative path that's outside of the currently opened zarr group? e.g., if I open a zarr group with an arbitrary store, and there's another group at "../../../", how do I get to that group?