ZEP9: Parse Metadata Objects

[brokkoli71]

This PR implements the following aspects of ZEP9 (Phase 1)

• If metadata JSON contains invalid keys, or if a value object contains invalid keys, the zarr array should be rejected unless it contains `"must_understand": false``.
• It should be possible to specify a value in metadata in the following ways
  • as a JSON object
  • As a string value (only if the object would have no attributes other than its name).

this will help to enable zarr extensions

TODOs in code:

• read metadata fails accordingly (+test)
• specifying metadata value works as string (+test)

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/user-guide/*.rst
• Changes documented as a new file in changes/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)