Using link-local addresses shouldn't require an address lot containing the address `::`

[jgallagher]

My naive attempts at using link-local uplink addresses failed due to this validation code on the insert path:

omicron/nexus/db-queries/src/db/datastore/address_lot.rs

Lines 340 to 354 in 1db108a

	// Ensure a lot block exists with the requested address.
	let block = block_dsl::address_lot_block
	.filter(address_lot_block::address_lot_id.eq(lot_id))
	.filter(address_lot_block::first_address.le(inet))
	.filter(address_lot_block::last_address.ge(inet))
	.select(AddressLotBlock::as_select())
	.limit(1)
	.first_async::<AddressLotBlock>(conn)
	.await
	.map_err(|_e| {
	ReserveBlockTxnError::CustomError(
	ReserveBlockError::AddressNotInLot,
	)
	})?;

@rcgoodfellow pointed me to a working config-rss.toml in a4x2: https://github.com/oxidecomputer/testbed/blob/14a77225190af5a41cb4f7f2d7f7785e9c840bf5/a4x2/config/rss-pieces/rack-network-bgp-unnumbered.toml

Critically, that config defines the infrastructure address lot as

infra_ip_first = "::"
infra_ip_last =  "::"

which allows us past the above address lot check. However, we're only using :: as a sentinel value as a way to say "use an automatically generated link-local address". We should either skip the address lot check for link-locals, or address lots should have a setting for "should link locals be allowed" and we check that instead.

This should be cleaner to implement after #10082 lands - we should be able to push an UplinkAddress down into address_lot::try_reserve_block() instead of an IpNetwork.