Selective read/write mudata modalities

[racng]

Is your feature request related to a problem? Please describe.
Reading and writing MuData is a bit slow sometimes. For example, after doing some TCR sequence analyses the MuData takes longer to read/write. Sometimes I added one annotation to mdata.obs but then it requires writing all modalities when saving. I appreciate that there is the ability to read and write one specific modality specified like mdata.h5mu/rna but there is no option to read and write only non-modality related elements like mdata.obs, mdata.var, mdata.obsm, etc. I imagine it could save time in different use cases.

Describe the solution you'd like
Ability to specify list of modalities to read/write, with the option to give an empty list such that only mdata non-modality related elements are read/written. This could be implemented by an extra argument in existing MuData IO functions.