Trigger jets: add a minimum pt threshold for b-tagging track association

This adds a minimum pt requirement to run the b-tagging track association on trigger jets. It's intended to make our track selection more robust to changes in the minimum jet building pt threshold.

I've set this threshold to 5 GeV for trigger, 4 GeV offline. This was chosen so that we don't change any outputs for now, but the threshold is accessible via a global flag so we can experiment easily.