Appends fun to the internal data-loader registry (session$state$data_loaders).
Registered functions are intended to be run sequentially in registration
order.
See also
Other fund/index workflow functions:
adjust_for_split(),
build_all_series(),
clear_data_loaders(),
clear_storage(),
get_storage(),
import_fund(),
join_env(),
run_data_loaders(),
store_timeseries()
