add function to generate visualization truth data `generate_truth_data.R`

[sbidari]

Adapt this code from covidhub, use pull_nhsn_hrd instead of pull_nhsn which is now deprecated

https://github.com/CDCgov/covid19-forecast-hub/blob/c46e0f06814c1a1349fdcdccc0a119cf2bfd541e/src/get_target_data.R#L1-L68