Libbeat disk queue should support custom serialization

[faec]

Currently libbeat's disk queue (de)serialization (https://github.com/elastic/beats/blob/main/libbeat/publisher/queue/diskqueue/serialize.go) assumes the underlying event type is publisher.Event, which needs to change before we can support the disk queue in the shipper. The disk queue should accept a serializer as part of its configuration (possibly defaulting to its current behavior for simplicity in libbeat), so we can use it to store shipper events.