Configurable merge distances

[xiyuoh]

This PR supplements #286 to add a few more configurable parameters to the fleet adapter:

• max_merge_waypoint_distance
• max_merge_lane_distance
• min_lane_length

As suggested, a default fleet value for each parameter can be configured in the fleet's config.yaml (example), and robot-specific distances can be set as well (example). To modify these values live, the EasyRobotUpdateHandle API is updated to accommodate getting and setting them.

These values are currently set independently since it might be more preferable to retrieve them individually, but I am wondering if it is better to take the params as optional values into a single function instead similar to RobotContext, such as

void EasyFullControl::EasyRobotUpdateHandle::set_nav_params(
  std::optional<double> max_merge_waypoint_distance,
  std::optional<double> max_merge_lane_distance,
  std::optional<double> min_lane_length)
{
  // modify value if has_value()
  // ...
  context->set_nav_params(new_nav_params);
}

[mxgrey]

Thanks for adding these parameters! We'll merge this in as soon as CI passes right away and let the CI run for the other PR.