Subnet: Show linked custom router

[askfongjojo]

The optional custom_router_id attribute is a key piece of information for the VPC router feature. It'll be very useful to show the name of the router in the subnet table.

[askfongjojo]

Two other possible behavior enabled once we show the custom router:

1. The custom router name can have a link to navigate user to the router page.
2. The "edit" popup allows user to set/unset the custom router value.

[david-crespo]

Custom router has to exist at subnet create/update time, so the input should be a listbox.

https://github.com/oxidecomputer/omicron/blob/6dd980251a26430466bcd5aff1edad5416cf94e5/nexus/src/app/vpc_subnet.rs?plain=1#L311-L324

[charliepark]

A few notes to self:

• will go in subnet-create and subnet-edit
• we'll need to clear the value; what should UI look for that?
• field label: "Custom router name"
• per David's note above, will need to be an existing custom router for the backend to handle it; API will catch this, so probably don't need to add specific validation
• router should be editable in subnet-edit, so they can switch to a different custom router

[charliepark]

(Got confirmation from Angela that many subnets can have the same custom router listed, so we don't need to prune the list based on already-committed routers.)