Create a field_storage_exists() function

Description of the need

In https://github.com/backdrop/backdrop-issues/issues/5170 We described how Backdrop allows field configs to exist without a corresponding field data table. A consequence of this situation is also discovered in https://github.com/backdrop/backdrop-issues/issues/3224#issuecomment-903... where if we attempt to save an entity with an attached field and instance, we get an exception.

Proposed solution

This new function would allow Backdrop to check if any storage method described in a field has been set up for that field. Currently in core, we only have field_sql_storage as a method

Alternatives that have been considered

We could also attempt to create storage automatically upon trying to save any attached field, in addition to this check