Do not pass CloudEvent type attribute via ceOverrides

[antoineco]

The two sources labeled on this issue hack the ceOverrides spec in order to override the type attribute of CloudEvents sent by their receive adapter:

https://github.com/triggermesh/event-sources/blob/7dfd216b238e65228c081729fe858c36065ce6a6/pkg/reconciler/azureactivitylogssource/adapter.go#L129-L132

This practice is strongly discouraged, and the only reason why it currently works is due to a bug in the client library:

When spec.ceOverrides.extensions is specified and non-empty, the event sent to the sink is augmented or modified with the specified extension attributes. The source controller MUST reject extensions overriding the required Cloud Events attributes.

-- https://github.com/knative/eventing/blob/5c08bd3d/docs/spec/sources.md#duckspec

We should fix this ASAP by using either

• message processors (e.g. setting it to "activity-logs" enables knowledge about Activity Logs payloads)
• a plain env var such as CE_TYPE, which may work when only 1 type is expected
• other suggestions

[antoineco]

Done in #423