Enable write to existing zarr store

[joshua-gould]

Added ability for write_zarr to accept a zarr group

[codecov]

Codecov Report

❌ Patch coverage is 62.50000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.95%. Comparing base (c9d6046) to head (90bcc92).
⚠️ Report is 106 commits behind head on main.

Files with missing lines	Patch %	Lines
src/anndata/experimental/backed/_io.py	25.00%	3 Missing ⚠️

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Flag	BASE (c9d6046)	HEAD (90bcc92)
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Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2366       +/-   ##
===========================================
- Coverage   87.35%   58.95%   -28.41%     
===========================================
  Files          49       49               
  Lines        7784     7776        -8     
===========================================
- Hits         6800     4584     -2216     
- Misses        984     3192     +2208     

Files with missing lines	Coverage Δ
src/anndata/_core/anndata.py	62.84% <ø> (-20.51%)	⬇️
src/anndata/_io/specs/registry.py	82.77% <100.00%> (-12.09%)	⬇️
src/anndata/_io/zarr.py	69.41% <100.00%> (-10.36%)	⬇️
src/anndata/experimental/backed/_io.py	37.25% <25.00%> (-59.11%)	⬇️

... and 28 files with indirect coverage changes

[ilan-gold]

@joshua-gould Awesome feature - could you look at

anndata/src/anndata/_io/specs/registry.py

Lines 389 to 397 in ef466f2

	if k == "/":
	if isinstance(store, ZarrGroup):
	from zarr.core.sync import sync
	sync(store.store.clear())
	else:
	store.clear()
	elif k in store:
	del store[k]

. This is just some old behavior that was never updated but I think this should be relaxed to just clear out all subgroups/keys instead of blindly clearing the entire store. I'm concerned about writing to an existing group but then you end up deleting all of its contents even those in "parent" groups

Other than that, this looks good to me!