Problem:

std_msgs/Float64MultiArray is deprecated.

# This was originally provided as an example message.
# It is deprecated as of Foxy
# It is recommended to create your own semantically meaningful message.
# However if you would like to continue using this please use the equivalent in example_msgs.

# Please look at the MultiArrayLayout message definition for
# documentation on all multiarrays.

MultiArrayLayout  layout        # specification of data layout
float64[]         data          # array of data

https://github.com/ros2/common_interfaces/blob/foxy/std_msgs/msg/Float64MultiArray.msg

Proposal:

Create (an) alternative in control_msgs. This could be a set of messages with a single joint interface type, one supporting multiples, or one message that has a field for the joint interface type and one that is fully dynamic --- (dynamic joint states??).