`BloscCodec` is broken

[d-v-b]

We need to fix the following behavior of the BloscCodec:

# /// script
# requires-python = ">=3.11"
# dependencies = [
#   "zarr@git+https://github.com/zarr-developers/zarr-python.git@main",
# ]
# ///
#
# This script automatically imports the development branch of zarr to check for issues

from zarr.codecs import BloscCodec
b = BloscCodec()
b.to_dict()
# ValueError: `typesize` needs to be set for serialization.

This means picking usable defaults instead of creating a completely broken object.

Unfortunately I am not sure how to gracefully fix this, since we have load-bearing internal code that relies on certain BloscCodec fields being set to None.

[d-v-b]

here is a concrete solution that would be backwards-compatible:

We add a new attribute to the Codec API that defines which of the other codec attributes can be re-set in the context of a codec pipeline. Codec creation would look like this:

# The itemsize can be tuned in the codec pipeline
compressor=BloscCodec(typesize=2, tunable={"itemsize"})

# The itemsize _cannot_ be tuned in the codec pipeline
compressor=BloscCodec(typesize=2, tunable={})

obviously evolve_from_array_spec would have to respect the contents of the tunable attribute