Array conversion methods

[slevang]

• relates to: Compatibility with the Array API standard  #7848
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

This implements a convenience method to convert data between duck array types, and another to check the current type of your data. Basically identical to cupy-xarray's as_cupy()/is_cupy() methods, but exposed in a more general way.

The signature I have here takes a callable plus any kwargs, so it's quite flexible:

ds.as_array_type(cp.asarray)
ds.as_array_type(jnp.from_dlpack)
ds.as_array_type(jnp.asarray, device=jax.devices("gpu")[0])
ds.as_array_type(pint.Quantity, units="m/s")

Then:

ds.is_array_type(cp.ndarray) # -> True

I'm not sure about the naming. There are also other ways we could go about this, e.g. string matching for supported arrays.

Follow up to #9798.

[slevang]

@shoyer @dcherian any further thoughts on this API?