[5/n] [sync-switch] extract db translation into a new crate

[sunshowers]

Begin introducing a new crate called nexus-switch-config-preparation, mirroring nexus-reconfigurator-preparation. For now this just consists of some simple helpers -- we're going to move the actual reads next.

This is a pure refactor with no functional changes.

Depends on:

• [1/n] [sync-switch] build bootstore bgp peers from already-loaded port settings #10642
• [2/n] [sync-switch] move bootstore config building into nexus-switch-config #10643
• [3/n] [sync-switch] move bfd and address lot reads back to bg task, make the builder pure #10644
• [4/n] [sync-switch] replace db-model builder inputs with domain types #10645