EventGsonProvider: Register generic event serializer adapter

This custom serializer is only needed if instance is created in one
plugin, but the serialization takes place in another plugin. If the
class is reachable from the same classpath loader where the event
instance was instantiated, then it will work without this change.

This will simplify multi-site plugin setup, that is using inter-plugin
communication:

o multi-site plugin, that is using events-broker
o events-broker libModule, that provides BrokerApi abstraction
o events-kafka plugin, that is using Apache Kafka event streaming
  backend and is used by events-broker's DynamicItem
o events-gcloud-pubsub plugin, that is using GCloud's PubSub event streaming
  backend and is used by events-broker's DynamicItem
o events-aws-kinesis plugin, that is using AWS's Kinesis event streaming
  backend and is used by events-broker's DynamicItem

This change would allow us to drop EventGsonProvider fork in multi-site
plugin.

Change-Id: I7dc6e6a859152ee3e30a90075ddd34814720eba1
(cherry picked from commit 89f7a75dbaebdb7bba92ae1f11c7505ec80dcfc2)