Rename CloudEvent in avro spec to AvroCloudEvent

[fabiojose]

Some users of Avro Event Format may use some kind of tool for source code gen, then the resulting source may collide with packages and classes defined in the SDKs.

This happens because of name definition within spec.avsc:

{
    "namespace":"io.cloudevents",
    "type":"record",
    "name":"CloudEvent"
}

The suggestion is to change the name to AvroCloudEvent, that I think is better for the context.

This is a result of experience using Avro with Java.