Support mandatory and logs-only modes for Check() adapters

[douglas-reid]

From a conversation with @wora this morning:

It should be possible to define Check adapters (and possibly Quota) that operate in a log-only like mode. In other words, an operator should be able to add adapters that execute on Check() calls but that don't impact the outcome of the call.

This would be to support the rollout / validation of new IAM checks, etc., without potentially causing outages. @wora mentioned support for "dark launches" when describing this use case.