`mgd` static routing API: restructure to reject nonsense path combinations?

Property-based tests hitting the mgd static routing API produced some API calls that tried to set up multiple overlapping routes; e.g.,

        v4: {
            "0.0.0.0/2": [
                Path {
                    bgp: None,
                    nexthop: 1.2.3.4,
                    nexthop_interface: None,
                    rib_priority: 14,
                    shutdown: false,
                    vlan_id: None,
                },
                Path {
                    bgp: None,
                    nexthop: 1.2.3.4,
                    nexthop_interface: None,
                    rib_priority: 153,
                    shutdown: false,
                    vlan_id: None,
                },
            ],
        },

mgd returned an OK for adding these routes, but internally only kept the priority 14 one. The current structure of this API is a HashMap<String, Vec<Path>> where the key is a stringified prefix. @taspelund noted that the more accurate structure here would be a Map<(Prefix, Nexthop, VlanId), Properties> - we can have at most one entry for that triple. JSON (and therefore OpenAPI) requires keys be strings, but we can still use stronger types on the Rust side.

A technique we've used in other places in Omicron is to make the OpenAPI / JSON type a flat Vec, so in this case a Vec<(Prefix, Nexthop, VlanId, Properties)>, but serialize/deserialize from the stronger Map type (be that a HashMap/BTreeMap or any other map/set-like collection), and have the Deserialize implementation raise an error if there are any duplicate keys.

Locate the mgd static routing API and its current HashMap<String, Vec> representation; the payload does not name files or tests. Start by tracing its Rust serialization and deserialization, then exercise the property-based duplicate-route case. Done means duplicate (Prefix, Nexthop, VlanId) entries are rejected instead of silently retaining one.