Add ftagfloat_t to centralize differences between 21.2 and master.

We'll eventually want to move all the double in flavor tagging to float. This is basically a cherry pick of !32782 (merged), with some merge conflicts fixed.

It's reasonable to ask why we don't just change the type of all the variables. Unfortunately we need this code to run in both 21.2 and master, and we're not going to change 21.2 and break everything without a lot more discussion. The idea here is to centralize the difference in one typedef, which is easy to change for compatibility between releases.

If we want to get very fancy we could also use some of the preprocessor macros that we use for duel use tools. But I'm not that fancy.

Also pinging @sanmay and adding a reference to ATLASRECTS-5201.