feat: allow writing into open HDF5 file

[flying-sheep]

I decided against allowing to pass an open (Python) file, since

1. what knobs would we use to wrap it in h5py.File? As seen in this PR, to open a BytesIO in memory, two parameters need to be passed to h5py.File
2. that would make tests like the concat dask test silently pass, as BytesIO is pickleable, so it’d just get transferred over the wire, while h5py objects aren’t pickleable so they crash Dask as expected.

TODO:

• test if this actually speeds up tests like @ilan-gold’s zarr variant of this

  /edit: no difference, so let’s forget about it.

---

• Closes #
• Tests added

• Release note not necessary because:

[codecov]

Codecov Report

❌ Patch coverage is 92.15686% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.85%. Comparing base (425e98e) to head (7f1090b).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/anndata/_core/anndata.py	77.77%	2 Missing ⚠️
src/anndata/_io/h5ad.py	86.66%	2 Missing ⚠️

Additional details and impacted files

@@                      Coverage Diff                      @@
##           ig/memory_store_for_tests    #2637      +/-   ##
=============================================================
+ Coverage                      85.83%   85.85%   +0.02%     
=============================================================
  Files                             49       49              
  Lines                           8133     8168      +35     
=============================================================
+ Hits                            6981     7013      +32     
- Misses                          1152     1155       +3     

Files with missing lines	Coverage Δ
src/anndata/_core/merge.py	86.23% <100.00%> (ø)
src/anndata/_io/specs/lazy_methods.py	96.62% <100.00%> (+0.09%)	⬆️
src/anndata/experimental/backed/_lazy_arrays.py	92.37% <100.00%> (ø)
src/anndata/tests/helpers.py	84.21% <100.00%> (+0.43%)	⬆️
src/anndata/_core/anndata.py	86.21% <77.77%> (-0.19%)	⬇️
src/anndata/_io/h5ad.py	92.13% <86.66%> (-0.56%)	⬇️

... and 1 file with indirect coverage changes