A description of a notification channel. The descriptor
includes the properties of the channel and the set of labels or fields
that must be specified to configure channels of a given type.
The type of notification channel, such as “email”, “sms”, etc.
Notification channel types are globally unique.
A human-readable description of the notification channel type.
The description may include a description of the properties of
the channel and pointers to external documentation.
The tiers that support this notification channel; the project
service tier must be one of the supported_tiers.
