Evolve the organically grown `::getClientSideInfo()` into something cohesive

Also I think we call this in one or two places and throw away most of the data it generates.. maybe consider splitting it up into methods for the individual parts if that seems viable.

#3++ — but here I'm trying to approach it from the other end: what information does the client actually need and what should it be shaped like?

BTW, now that 📌 Expand `::getClientSideInfo()` test to all ComponentSource plugins Active is in, writing test coverage for this has become zero effort — all we need to do is update the existing expectations 👍

(Also: 📌 [PP-1] Rename PropSourceComponent.field_data to propSources Postponed is already in! 👍)