[3/n] [sync-switch] move bfd and address lot reads back to bg task, make the builder pure

[sunshowers]

Move the infra address-lot and BFD reads back into the caller, passing their results in via a new RackNetworkConfigInput struct. With those gone, the builder performs no database access at all, so it becomes a synchronous, pure function.

The nexus-switch-config crate still has dependencies on nexus-db-model and nexus-db-queries, but those are only for types. Later commits in the series will introduce domain types and get rid of this dependency.

This commit reorders some reads slightly, but otherwise is meant to be pure refactoring 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