feature(store): add get_metadata and set_metadata methods to stores

[jhamman]

This is pointing at @d-v-b's fork because I branched of v3_group_tests. Once that PR goes in, I'll set the target to the zarr-python v3 branch.

Description: This PR adds two new methods to the v3 Store API (get_metadata and set_metadata). For most stores, this means that JSON serialization now happens in the Store. Crucially, this unlocks the ability for some stores to keep metadata in a container other than JSON (e.g. in a database that natively stores dictionaries).

Xref: zarr-developers#1686 (comment)

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)