Allow to inject failures in FDB mock admin client

[johscheuer]

What would you like to be added/changed?

This would allow to ensure we are resilient against different errors. We could start with allowing to inject a timeout error based as a fixed condition and to inject timeouts based on a probability (which can be set or be disabled (by setting the probability to 0)).

[johscheuer]

Initial setup: https://github.com/FoundationDB/fdb-kubernetes-operator/pull/1489