kvserver: establish per-store rangefeed over system.span_configurations

[irfansharif]

Part of #66348. We'll later use it to maintain an in-memory cache/tree of spans and the corresponding span configs (#67780).

[blathers-crl]

Hi @irfansharif, please add a C-ategory label to your issue. Check out the label system docs.

While you're here, please consider adding an A- label to help keep our repository tidy.

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is otan.}

[irfansharif]

#66892.