Emit Events for BuildRun objects

[adambkaplan]

Idea:

The build controller should emit Kubernetes events for BuildRun objects. These should reflect the important state transitions for the BuildRun's lifecycle - namely if it started, if it succeeded, or if it failed.

The set of events we emit to the cluster can be the building blocks for events that can be emitted outside the cluster, through specifications like CloudEvents.

/kind feature