Use `NameOrId` in `InstanceCreate.external_ips`

[david-crespo]

#3985 (comment)

I think the main reason it was name only was that the name "default" was significant, and now it isn't.

omicron/nexus/types/src/external_api/params.rs

Lines 840 to 846 in 58e8c67

	/// The external IP addresses provided to this instance.
	///
	/// By default, all instances have outbound connectivity, but no inbound
	/// connectivity. These external addresses can be used to provide a fixed,
	/// known IP address for making inbound connections to the instance.
	#[serde(default)]
	pub external_ips: Vec<ExternalIpCreate>,

omicron/nexus/types/src/external_api/params.rs

Lines 802 to 811 in 58e8c67

	/// Parameters for creating an external IP address for instances.
	#[derive(Clone, Debug, Deserialize, Serialize, JsonSchema)]
	#[serde(tag = "type", rename_all = "snake_case")]
	pub enum ExternalIpCreate {
	/// An IP address providing both inbound and outbound access. The address is
	/// automatically-assigned from the provided IP Pool, or all available pools
	/// if not specified.
	Ephemeral { pool_name: Option<Name> },
	// TODO: Add floating IPs: https://github.com/oxidecomputer/omicron/issues/1334
	}