Add ROS2 array ("sequence<X>") support

[egordon]

In response to issue #23.

Unfortunately, get_slot_type in rqt_py_common is now out-of-date, as it doesn't handle sequence types. This PR by-passes that function to allow rqt_publisher to publish array-like messages of type "sequence".

This is done by:

• Manually catching sequence types (which are represented as list)
• Manually catching all sequence types (which are represented as array.array)
• Modifying the eval expression to allow for defining array literals
• Finally, since array.array does not have a copy constructor, allow for the passing of the direct evaluation if the type matches.

Note: this has only been tested on my personal setup with sensor_msgs/msg/JointState, as I am unsure of a better testing suite.

[egordon]

Wanted to bump @gbiggs

We've been using our own copy of this repo for months now to publish arrays, wanted to see if you want this upstream merge (happy to put in cycles to fix it if needed) to help others with #23

[styczen]

Little bump for @gbiggs @egordon
It seems that this problem is already solved with #33 but this is for Rolling. To use it on Humble, one has to build this plugin from source.