Using entry_points to register dataset and dataarray accessors?

Is your feature request related to a problem?

External libraries often use the dataset/dataarray accessor pattern (e.g. metpy). These accessors are not available until importing the external package where the registration occurs. This means scripts using these accessors must include an often-unused import that linters will complain about e.g.

import metpy # linter complains here

# some data
ds: xr.Dataset = ...

ds.metpy....

Describe the solution you'd like

Use importlib entrypoints to register these as entrypoints so that registration is automatically handled. This is currently enabled for the array backend, but not for accessors (e.g. metpy's setup.cfg).

Describe alternatives you've considered

No response

Additional context

No response

Start by reading the existing importlib entrypoint handling for array backends and compare it with the accessor registration pattern described in the issue. Review MetPy's setup.cfg entrypoint example and determine how Dataset and DataArray accessors should be discovered and registered without an explicit external-package import. Done means an external accessor is available through the entrypoint mechanism and the relevant behavior is covered by tests.