[7/n] [sync-switch] add single-transaction switch config read (not yet wired up)

[sunshowers]

This is part one of the Nexus-side fix for #10640 -- it adds a way to read the entire switch config in a single transaction.

This isn't actually used yet -- the next commit in this series will start using it.

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
• [5/n] [sync-switch] extract db translation into a new crate #10646
• [6/n] [sync-switch] factor switch_port_settings_get body out #10647