Expose Machine Liveliness in OpenAPI/Swagger

[vknabel]

Currently the enum for possible liveliness are not exposed and reside within the internal folder. They should be available as enum in metal-go.

const (
	MachineLivelinessAlive   MachineLiveliness = "Alive"
	MachineLivelinessDead    MachineLiveliness = "Dead"
	MachineLivelinessUnknown MachineLiveliness = "Unknown"
	MachineDeadAfter         time.Duration     = 5 * time.Minute
	MachineResurrectAfter    time.Duration     = time.Hour
)