feat: "backed" mode for read_lazy

[ilan-gold]

The big open question is what is backed mode even supposed to be at this point? Right now we don't attach an "open" dataset to X but instead reopen every time (which is presumably for a good reason? open file handles?). So the boolean for indicating backed mode chekcs that X is None, which has a knockdown list of side effects.

In general, I think a plan of attack:

• Figure out why old backed mode reopens the file repeatedly
• Does having a write mode make sense in the age of copy-on-write i.e., adata[subset].X = ... doesn't write to disk?
• Based on that, either attach the zarr.Group necessary for reopening or get rid the concept of reopening if possible (preferably the later)
• Coordinate the result of the above with feat: Unify X and layers #1707
• Decide what AnnData.file{name} even should be for in the wake of this

---

• Closes read_lazy kwarg for not using dask #2147
• Tests added

• Release note not necessary because:

[codecov]

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