* Add the ability to log all engine events to a file.
The path to the file can be specified using the `--event-log` flag to
the CLI. The file will be truncated if it exists. Events are written as
a list of JSON values using the schema described by `pkg/apitype`.
* Expose update engine events to ExtraRuntimeValidation.
Just what it says on the tin. Events from previews are not exposed.