Move instance networking functions into their own module

[gjcolombo]

nexus/src/app/instance.rs and sled.rs implement several networking-related functions that are less directly concerned with instance or sled management than their sibling routines. Tidy things up a bit by creating an instance_network module and moving instance V2P and NAT management functions there. Also, move instance NAT entry deletion logic into its own function that's called from the instance delete saga instead of implementing it inline in that saga.

These changes aim to reduce clutter in instance.rs and to move NAT entry deletion to a function that can be reused by subsequent changes to the way Nexus handles instance stop.

Except for some minor edits to error handling in instance_delete_dpd_config (needed because this function no longer returns a steno::ActionError), this PR only rearranges existing code and has no functional changes.

[smklein]

I checked that functions signatures look the same (and they do, with the exception of the new function) but not that the bodies are the same. Trusting you that they are!

Thanks for the cleanup.