Port HGTD fast digi from 21.9 to master
This MR follows !45088 (merged) as the next step in getting HGTD fast digi up and running in master. These developments were first introduced in 21.9 through !43255 (merged).
Note that these changes don't include anything yet on the configuration side, which will come in a follow-up MR.
Edited by David Richard Shope