Ignore v3 metadata members declaring must_understand: false - #94

A Zarr v3 writer may add members to a metadata document that a reader does
not know about. When such a member is a JSON object carrying
"must_understand": false, the specification requires a reader to ignore it;
any other unknown member has to be rejected, because it may change how the
array is to be interpreted.

zarr-java rejected every unknown member instead, because the ObjectMapper in
v3.Node was built with Jackson's default FAIL_ON_UNKNOWN_PROPERTIES. Reading
a zarr.json written by a newer implementation therefore failed with an
UnrecognizedPropertyException on a member that was explicitly marked as safe
to skip.

Unknown members are now collected through a @JsonAnySetter creator parameter
and validated in ExtraFields, which mirrors zarr-python 3.1.6
(zarr/core/metadata/v3.py):

  - a JSON object with "must_understand": false is ignored,
  - anything else is rejected with a ZarrException,
  - an extra field colliding with a member the document defines itself is
    rejected.

Ignored members are kept on the metadata and written back out by a
@JsonAnyGetter, so that another implementation's extension survives a
metadata rewrite. ArrayMetadataBuilder.fromArrayMetadata and
Group.setAttributes carry them over, which covers resize, setAttributes and
updateAttributes.

must_understand deliberately does not apply to codec, chunk grid or chunk key
encoding names: a codec cannot be skipped and still leave the chunk bytes
decodable, so an unknown name there still fails.

Verified against zarr-python 3.1.6 in both directions: a document it writes
with an extra field now opens, and a document zarr-java rewrites is read back
by zarr-python with the field intact in its own extra_fields.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>