Generically expose URDF joint dynamics.

[danzimmerman]

This is aimed at simulation use cases like #54 and #55, where simulation needs dictate different non-zero joint dynamics parameters.

In ur.urdf.xacro I provide an example loading the joint dynamics properties from a config file, but followed the treatment of initial_positions so that no config file is necessary to use the xacro:ur_robot macro in other contexts.

For the Gazebo Classic use case, I want to compute the dynamics parameters dynamically from other data, setting the joints' friction attributes to a multiple of the joints' effort limits like here.

(See discussion in #55 and Universal_Robots_ROS2_Gazebo_Simulation/#19 for context.)

In other user code including ur_macro.xacro I think this should not require any changes.

[danzimmerman]

This should likely wait and build on top of #59 or equivalent.