Allow `cls` to be a string of type class name in `DataStore._is_subclass()`

[mylibrar]

Is your feature request related to a problem? Please describe. Currently DataStore._is_subclass(type_name, cls) require input argument cls to be an entry class. We also want it to support string of fully qualified name of the entry (e.g., "forte.data.ontology.top.Annotation").

Describe the solution you'd like Maybe add some type checking for cls and convert it to a class using get_class() when it's a string.

[hunterhector]

@J007X this is a potential place for improving efficiency