Add `CompleteUserTask` to Audit Log

[Snarr]

In user_tasks.proto, we have a message UserTaskEvent that is stored in the audit log of a UserTaskRun for observability purposes. There are different types of UserTaskEvents that can be logged, and we should add SaveUserTask and CompleteUserTask to this list of types once they are ready.

From schemas/littlehorse/user_tasks.proto:

message UserTaskEvent {
  ...
  oneof event {
    // Denotes that a TaskRun was scheduled via a trigger.
    UTETaskExecuted task_executed = 2;

    // Denotes that the UserTaskRun was assigned.
    UTEAssigned assigned = 3;

    // Denotes that the UserTaskRun was cancelled.
    UTECancelled cancelled = 4;
    // TODO: Add "save user task" and "complete user task" to the
    // audit log
  }
}

[Snarr]

Partially resolved: https://github.com/littlehorse-enterprises/littlehorse/pull/946