Provides a subscribable state source for a "perspective" for the Sanity client,
which is used to fetch documents as though certain Content Releases are active.
Param: instance
The Sanity instance to get the perspective for
Param: options
The options for the perspective -- usually a release name
Returns
A subscribable perspective value, usually a list of applicable release names,
or a single release name / default perspective (such as 'drafts').
Provides a subscribable state source for a "perspective" for the Sanity client, which is used to fetch documents as though certain Content Releases are active.