Creates a new DomainMappingSpec.
certificateMode The mode of the certificate.
forceOverride If set, the mapping will override any mapping set before this spec was set.
routeName The name of the Cloud Run Service that this DomainMapping applies to.
If set, the mapping will override any mapping set before this spec was set.
It is recommended that the user leaves this empty to receive an error
warning about a potential conflict and only set it once the respective UI
has given such a warning.