implement user creation strategies

this rather large MR implements a new framework for strategies how we create users from the helmholtz AAI via the HELMHOLTZ_CREATE_USERS_STRATEGY. It's supposed to replace the settings HELMHOLTZ_MAP_ACCOUNTS and HELMHOLTZ_CREATE_USERS. the latter makes it too complicated to implement various strategies, so I blow it up a bit. the envisaged strategies can be reviewed at https://notes.desy.de/jaTtlwDBSVmo5mls5Uz2oA?both#