[v3] Feature: Store open mode

[jhamman]

This adds a new required attribute/property to the store API (Store.mode). Enforcing read/write permissions at the store is a much cleaner way of managing access than at the Group/Array level (as it is done in v2). I haven't pushed any test changes yet (waiting on #1900) but those will come next.

Xref: #1686 (comment)
Toward #1755

Design note: Think of this as the MVP, with the main focus on the Store.mode property and the most basic functionality. I think we can continue to explore how to help stores enforce permissions (e.g. via a decorator or something else) later.

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)

[jeromekelleher]

Lgtm!