Add an option to save the current H5P content state (to recover it later) for the H5P activities.

This implementation requires:

• Add support to xAPI State in core_xapi,  in order to save the content state.
• Implement/override the required classes/functions to use xAPI state get/post/delete inside mod_h5p.
• Add support for state saving and restoration from the H5P player.
• Behat tests for saving and restoring states inside an H5P activity

Things to consider:

• Backup/restore
• Reset the state when the H5P activity content is changed
• Implement methods from the framework related to the save content feature. That is:
  • resetContentUserData (called from saveContent)
  • deleteContentData (called from deletePackage)
• Setting to let admins define the frequency to save the user content (saveFreq). The saveFreq H5P setting is also used to trigger the methods required to save the content state.
• A mechanism to clean old entries in the xapi_states table (based on an admin setting to let them define how long should these entries be stored).

More information:

• General information about xAPI state: https://learningpool.com/xapi-state-api/ and https://xapi.com/blog/deep-dive-state-activity/ 
• xAPI specification: https://github.com/adlnet/xAPI-Spec/blob/master/xAPI-Communication.md#23-state-resource 
• Query example xAPI state: https://www.xapijs.dev/xapi-wrapper-library/state-resource 
• Information about statements: https://xapi.com/statements-101/
• H5P doesn't support State API: https://h5p.org/node/468797