Expose rate configuration

[tcr3dr]

See de461b6.

1. Does this need to be included as a connect param at all? i.e. Can this be done exclusively in a follow up configuration?
2. What happens if None is passed, is there a "default"?
3. What would a standalone vehicle.request_rate function look like and act like?

[hamishwillee]

Is this rate set using REQUEST_DATA_STREAM?

I don't know much about this, but my "understanding" is that all these will have defaults as per here - perhaps check with @RMACKAY ?. That means that it would not need to be in the connect param, and the default would be not to send the information :-)

Another consideration is that it looks like REQUEST_DATA_STREAM will be replaced by a per-message interval DATAMAV_CMD_SET_MESSAGE_INTERVAL. It is also possible to set stream rates using SR1_* and SR2_* parameters - the message option is not persistent across reboots.