Simplesingletable

Added

* Added a new "extras" sub-package; added new Singleton resource models for storing / retrieving things that should only
be in the database once, like application configuration.
* Added a "use_case_examples" top-level folder with examples of various features and access patterns; currently includes
a single script for the Singleton objects.

Added

* All resource types now support overriding the default gsitype sk value (`updated_at.isoformat()`) via the
method `db_get_gsitypesk` to enable alternative access patterns on the `gsitype` index.

Added

* Added `delete_existing` method for non-versioned resources.

Changed

* Breaking change; switched to the full class name by default for the custom resource identifier, rather than just the
extracted capital letters.

Added

* Add `resource_config` ClassVar to begin exposing configuration of resources.

Changed

* Renamed models and memory class for consistent casing convention; left backwards compatible names in place as well.

Fixed

* Pinned `pydantic` to a minimum working version.