Make prefix configurable for the data stored in the metadata bucket

We need to add a field in the UI to configure the prefix for the data stored in the metadata bucket. The prefix is the handler name, but it's not sufficient as we need a more mechanical way of calling up all of eventing data separate from user data.

We should:
(a) Ensure the first part of the prefix, by default, is specific to the cluster – like a hash of cluser ID
(b) Cluster specific prefix should be available for manipulating over REST API
(c) Export should not include cluster specific prefix

So prefix would be: <user prefix>:<cluster prefix>:<handler name> – the user prefix is important to be the first element, because the user prefix is what permits us to own a certain portion of the keyspace of a bucket (until we have collections). It is best if user prefix is specified alongside the metadata bucket choice on the UI.