(feat): read_lazy for whole AnnData lazy-loading + xarray reading + read_elem_as_dask -> read_elem_lazy

[ilan-gold]

This PR is a lighter weight version of #947 that involves using the original AnnData object as the class to hold obs and var xr.Dataset.

• Closes Dask and Zarr not loading obsp and obsm from remote s3 #951 and closes lazy dataframes in .obs and .var with backed="r" mode #981
• Tests added
• Release note added (or unnecessary)

[codecov]

Codecov Report

Attention: Patch coverage is 91.95876% with 39 lines in your changes missing coverage. Please review.

Project coverage is 84.17%. Comparing base (935cdf2) to head (b46ae71).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/anndata/experimental/backed/_lazy_arrays.py	94.17%	6 Missing ⚠️
src/anndata/tests/helpers.py	73.91%	6 Missing ⚠️
src/anndata/_core/storage.py	50.00%	5 Missing ⚠️
src/anndata/experimental/backed/_io.py	90.00%	5 Missing ⚠️
src/anndata/experimental/backed/_xarray.py	92.53%	5 Missing ⚠️
src/anndata/_io/specs/lazy_methods.py	93.93%	4 Missing ⚠️
src/anndata/_core/index.py	66.66%	3 Missing ⚠️
src/anndata/experimental/backed/_compat.py	84.21%	3 Missing ⚠️
src/anndata/_io/specs/registry.py	88.23%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1247      +/-   ##
==========================================
- Coverage   86.11%   84.17%   -1.94%     
==========================================
  Files          40       45       +5     
  Lines        6242     6671     +429     
==========================================
+ Hits         5375     5615     +240     
- Misses        867     1056     +189     

Files with missing lines	Coverage Δ
src/anndata/_core/aligned_df.py	95.83% <100.00%> (+0.18%)	⬆️
src/anndata/_core/anndata.py	82.65% <100.00%> (+0.04%)	⬆️
src/anndata/_core/merge.py	85.14% <100.00%> (-8.75%)	⬇️
src/anndata/_core/views.py	85.40% <100.00%> (-5.35%)	⬇️
src/anndata/_io/specs/__init__.py	100.00% <ø> (ø)
src/anndata/_io/specs/methods.py	88.36% <100.00%> (-0.41%)	⬇️
src/anndata/_io/zarr.py	83.75% <100.00%> (+0.20%)	⬆️
src/anndata/_types.py	86.11% <100.00%> (+0.81%)	⬆️
src/anndata/experimental/__init__.py	100.00% <100.00%> (ø)
src/anndata/experimental/backed/__init__.py	100.00% <100.00%> (ø)
... and 10 more

... and 3 files with indirect coverage changes

[ilan-gold]

@ivirshup @flying-sheep Not really looking for a thorough code review at the moment, more of a look at the structure of what we are exporting. The big changes are

1. read_elem_as_dask->read_elem_lazy becomes a more general method focused on supporting the reading of obs and var lazy as xarray objects (although now it can read categoricals, nullables, etc. as a side-effect)
2. read_backed is exported as a one-stop-shop for reading everything at once backed, as possible

Do we want this way of doing things? Or is there some other route?

Separately, are the changes made to the core acceptable? After that, I think we can look into the specifics of the code I added. Or you can review that now, but I'd rather get big changes out of the way first.

[ilan-gold]

I will continue to make little changes to clean things up (this is still a draft!) but I think this structure is the way I would go. But maybe you have different ideas!

[flying-sheep]

wait, I thought this doesn’t work. did they change that?

[ilan-gold]

What didn't work? Importing from `conftest?