Missing xAPI get states webservice needed for mobile APP

Since MDL-67789, moodle provides a core xAPI state implementation. This implementation covers the following webservices:

• get_state: to get the internal data of a state
• post_state: to store a new state data
• delete_state: to delete a stored state

All webservices follow the xAPI specs . However, there's a missing webservice required to support offline activity in the mobile APP:

• get_states: to get all the state ids created since a specific date.

This webservice is also part of the xAPI standard but missing from the code implementation.

Technical shaping

Extra documentation on the get states webservice:

• https://www.xapijs.dev/xapi-wrapper-library/state-resource#getstates
• https://learninglocker.atlassian.net/wiki/spaces/DOCS/pages/109772825/xAPI+States#Retrieve-Many-State-Identifiers

The implementation, in this case, is pretty easy as it follows almost the same structure as the existing get_state.