protobuf processor: buf schema registry (bsr) should allow the use of a cache #861

The protobuf processor allows users to configure a bsr object field that enables the polling of a 'buf schema registry'.

Considerable numbers of SchemaWatchers could be created by this processor, (processing threads / streams_mode / k8s pods etc.), it is necessary to allow users to make use SchemaWatcher's functionality pertaining to Caching.