Add jumbo frame support for control plane services

[rcgoodfellow]

Per the comment here:

omicron/sled-agent/src/services.rs

Lines 1180 to 1190 in f331e2d

	// TODO(RFD 689): plumb the fleet-wide jumbo-frames opt-in
	// (`system_networking_settings.external_jumbo_frames_opt_in_enabled`)
	// through blueprints / reconfigurator-execution to this point
	// so that external-facing service zones (Nexus, ExternalDns,
	// BoundaryNtp) are brought up with `EXTERNAL_JUMBO_FRAMES_MTU`
	// when the operator has enabled the opt-in.
	//
	// For now, services always use the default MTU. The
	// per-instance jumbo opt-in (the primary user-facing feature
	// from RFD 689) is fully wired through `instance_ensure_registered`.
	mtu: None,

In #10472 we did not plumb jumbo frame support for control plane services. We want to get a bit of experience with things with instances before we move on to services. This is a tracking issue for that.