Add layer parameter to read_10x_* functions #4071

What kind of feature would you like to request?

Additional function parameters / changed functionality / changed defaults?

Please describe your wishes

That can switch to be "counts" by default in 2.0.

TODO: should this be an AdRef instead? Probably not as only layer really makes sense, but will it be confusing if some functions keep parameter names named after anndata attributes? I guess we should first design the new “in/out” parameters before committing