Add some v2 proxies to pr tracks, turn Tag namespace into struct

MR includes two changes that are aimed at making the usage of the v2::SOACollection-based track containers easier.

1. turn the definition of the fields from namespace Tag { into struct Tag { and add a traits struct tag_type_t which is aimed to make generic programming easier for these containers.
   E.g. in the track fit where I template over input type I can now get the tag struct via using tag = tag_type_t<InputTracksType>

2. add some minimal v2::proxies to our the track containers such that I don't even need to know the tag or other details to ask a track how many hits it has etc.

Motivation is pretty much diffs like this:

Requires changes in Rec!2445 (merged)

fyi: @decianm @peilian @gunther @ahennequ