Add user-agent logging for server-side events

* A separate patch will update the server-side code to the latest
  EventCapsule schema.
* The field is artificially removed during parsing, to prevent
  validation failures.
* Includes tests for schema validation of non-required and required fields.

Change-Id: I79eab129f38fc70e03ee2a1eba0b3e5939f7312b