List all partition keys of an application

**Is your feature request related to a problem? Please describe.**
Right now, burr does not provide an interface to list all `partition_key`s of an application.

Quoting from the [documentation](https://burr.dagworks.io/concepts/state-persistence/#state-keys):

> In the case of a chatbot, the app_id could be a uuid, and the partition_key could be the user’s ID or email, etc. Note that partition_key can be None if this is not relevant. A UUID is always generated for the app_id if not provided.

Imagine we are implementing an admin page for the system managing the chatbot, and need to display all the users' chat sessions, that the interface to retrieve all the `partition_key`s from an application should become necessary.

**Describe the solution you'd like**
Maybe a method from the application to retrieve all the `partition_key`s?

**Describe alternatives you've considered**
Direct data retrieval from the storage where the states of the application are persisted.

**Additional context**
None