Simplified CFDP

A simplified CFDP implementation based off cfdp-rs.

Deviates from the CFDP Spec in the following ways:

- Uses fixed length entity IDs and transaction sequence numbers in the CFDP header
  - source_entity_id: u16
  - destination_entity_id: u16
  - transaction_sequence_number: u32
- CCSDS version in CFDP header is always 0
- Utilizes NAKs instead of finished and Ack PDUs
  - When receiver receives an EOF PDU, rather than sending an Ack(EOF) followed by a Nak(missing-data-segments), the receiver only sends a Nak(missing-data-segments)
  - When the receiver has received all data PDUs, instead of sending a Finished PDU and the sender following with an Ack PDU, the receiver sends an empty Nak PDU