Manila uses API microversions to allow natural evolution of its REST APIs
over time. But microversions alone cannot solve the question of how to
ship APIs that are experimental in nature, are expected to change at any
time, and could even be removed entirely without a typical deprecation
period.
Working in conjunction with microversions, this commit adds a facility
for marking individual REST APIs as experimental.
Implements bp: manila-experimental-rest-apis
Change-Id: I263a0b5579a7eb2fe98ca810ad3dec6719d66e6f
