New "mongodb.collection change event" types

[selim5]

Let's change the type to be something like: google.com.mongodb.collection.v1.inserted for inserts, google.com.mongodb.collection.v1.deleted for deletions, and google.com.mongodb.collection.v1.updated for updates.

And let's add them to the registry annotation in the yaml, similar to https://github.com/google/knative-gcp/blob/master/config/core/resources/cloudstoragesource.yaml#L23

Originally posted by @nachocano in https://github.com/googleinterns/knative-source-mongodb/pull/42