[WIP] Moving Cyphal/UDP to multicast

[maksimdrachov]

This MR is based on this proposol discussed in the OpenCyphal forum.

In short, the changes can be broken down into three pieces:

1. Datagram header format

Current:

uint8 version = 0
uint8 priority
void16 
uint32 frame_index_eot
uint64 transfer_id
void64

Proposal:

uint8 version = 1   # UPDATE
uint8 priority
uint16 source_node_id   # NEW
uint32 frame_index_eot
uint64 transfer_id
void64

Note: version will be bumped to 1, however no backward-compatibility changes are made (since the protocol is still in development).

2. Message

Current:

    fixed         reserved
   (9 bits)       (3 bits)
   ________          _
  /        \        / \
  11101111.0ddddddd.000sssss.ssssssss
  \__/      \_____/    \____________/
(4 bits)    (7 bits)      (13 bits)
  IPv4      subnet-ID     subject-ID
multicast   \_______________________/
 prefix             (23 bits)
            collision-free multicast
               addressing limit of
              Ethernet MAC for IPv4

Proposal:

    fixed   message  reserved
   (9 bits) select.  (3 bits)
   ________   res.|  _
  /        \     vv / \
  11101111.0ddddd00.000sssss.ssssssss
  \__/      \___/      \____________/
(4 bits)   (5 bits)       (13 bits)
  IPv4     subnet-ID      subject-ID
multicast   \_______________________/
 prefix             (23 bits)
            collision-free multicast
               addressing limit of
              Ethernet MAC for IPv4

3. Service

Current: regular unicast

Proposal:

    fixed          service
   (9 bits)  res.  selector
   ________      ||
  /        \     vv
  11101111.0ddddd01.nnnnnnnn.nnnnnnnn
  \__/      \___/   \_______________/
(4 bits)   (5 bits)     (16 bits)
  IPv4     subnet-ID     node-ID
multicast   \_______________________/
 prefix             (23 bits)
            collision-free multicast
               addressing limit of
              Ethernet MAC for IPv4

TODO

• [x] Datagram
• [x] Message
• [x] Service
• [x] Implement Scott's changes
• [x] Add anonymous mode
• [x] Update documentation