VPC and subnet name should be required in NIC create form

[david-crespo]

Bug noticed while reviewing #985 but not actually introduced by that PR.

These lines in the mock NIC create endpoint (which were already there) mean that any NIC create without a VPC name and a subnet name will 404.

console/libs/api-mocks/msw/handlers.ts

Lines 491 to 499 in 1f746d5

	const [vpc, vpcErr] = lookupVpc({ ...req.params, vpcName: vpc_name })
	if (vpcErr) return res(vpcErr)
	const [subnet, subnetErr] = lookupVpcSubnet({
	...req.params,
	vpcName: vpc_name,
	subnetName: subnet_name,
	})
	if (subnetErr) return res(subnetErr)

Turns out these are not actually optional on the POST, which is a relief because I did not understand how that would work. So those fields should be marked required in the create form.

https://github.com/oxidecomputer/omicron/blob/a0e5d59a3190b40138c5d6928f6cb89aebb00e9b/nexus/src/external_api/params.rs#L275-L287

[david-crespo]

I think the reason I didn't do this originally is that our listboxes doesn't enforce requiredness, so that is a bit of additional work needed here.