Add expected message frequency field

[simoneruffini]

• This field will describe the expected frequency at which a message is sent on CANbus.
• Used by either of both sender and receiver.
• This field could be a signed value that expresses the frequency in Hz (range -1 to 1'000'000)
• A special value (-1) denotes that the message will not have a frequency but instead burst after an undefined trigger.
• This value is useful so generation to a header file is a must.

[simoneruffini]

Col cavolo che lo metti a low priority