aucoalesce: Make warning capturing configurable

The current interface to aucoalesce is `func CoalesceMessages(msgs []*auparse.AuditMessage) (*Event, error)`.

Given that the warnings produced by this function are mainly for debugging and development we could change the function to accept a variable number of options like `func CoalesceMessages(msgs []*auparse.AuditMessage, opts ...Option) (*Event, error)`. Then we could pass an option to enable the inclusion of warnings with the event. Like

```golang
evt, err := aucoalesce.CoalesceMessages(msgs, aucoalesce.IncludeWarnings)
```

Future options might be to make ECS enrichment configurable.