Create monitor for alarming on cache drift

[jyotimahapatra]

While deploying xdsrelay in staging environment for a few critical services, we realized that the cache for eds had a drift.
The cache drift perhaps happened due to a bug. We need a way in xdsrelay to compare cache entried periodically and alarm when they dont match. The xdsrelay operators need a way to at least restart the server and reset the cache in order to mitigate when this happens.
There could always be bugs during initial launch, and such a method can keep the system safe. This feature should be based on a bootstrap config so that it can be enabled only when configured. It also needs a way to transform xdsrelay keys with upstream control plane keys.

[jyotimahapatra]

@jessicayuen We need some kind of monitor to make sure we alarm when cache drifts. We can make such a mechanism inhouse or think about this feature in the open from the outset, since this could be a helpful feature for operators to safely rollout the service. wdyt?