Add default compressors to config

[brokkoli71]

This PR adds:

• default compressor for zarr_format=2
• specification of default compressors dependent on dtype in zarr.config

fixes #2267

Should _get_default_array_bytes_codec for zarr_format=3 also be configurable in zarr.config?

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/tutorial.rst
• Changes documented in docs/release.rst
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[brokkoli71]

I am a little confused: in which cases should we use VLenBytes and in which cases VLenUTF8? Or do we need both at the same time?

[brokkoli71]

Also, do we need a default compressor or are default filters sufficient?