Add a new exporter type: DirectFLP (#223)

* Add a new exporter type: DirectFLP

DirectFLP starts the whole flowlogs-pipeline process directly from the
Agent, eliminating the need for a network connection between them.

This can be useful when both Agent and FLP run on the same host, saving
resources. This is mutually exclusive with other exporters such as with
Kafka.

rollback e2e deps bump

Avoid pbuf conversion when using direct-flp

Clarify test file name, use existing ipv6 const

Restore L2 drops

* update flp dep