Added ROS2 ActionClient w/ limited functionality

[danmartzla]

Added the roslibpy.ActionClient object to send actions goals to ROS2-based systems.
This implementation has limited capabilities (related to RobotWebTools/rosbridge_suite#909) such as:

• Goal cancelation is not functional.
• Goal rejection/acceptance are not being notified to the Action client.

What type of change is this?

• Bug fix in a backwards-compatible manner.
• New feature in a backwards-compatible manner.
• Breaking change: bug fix or new feature that involve incompatible API changes.
• Other (e.g. doc update, configuration, etc)

Checklist

• I added a line to the CHANGELOG.rst file in the Unreleased section under the most fitting heading (e.g. Added, Changed, Removed).
• I ran all tests on my computer and it's all green (i.e. invoke test).
• I ran lint on my computer and there are no errors (i.e. invoke check).
• I have added tests that prove my fix is effective or that my feature works.
• I have added necessary documentation (if appropriate)

[sea-bass]

This all looks great to me -- just a few minor formatting things and I think this V1 support will be a great addition. Thank you!

[danmartzla]

This all looks great to me -- just a few minor formatting things and I think this V1 support will be a great addition. Thank you!

Hi! I already fixed your suggested minor changes. Thanks.

[sea-bass]

Sorry, but I'm not a maintainer of this repo so I can't actually approve your changes. I was just looking because I implemented the rosbridge side of this work.

[sea-bass]

Sorry @danmartzla, I forgot to mention that I recently landed a small change to the protocol that allows for cleaner action cancellation. It may require a small change since now the action_result message also contains a status field telling you whether the action was successful, canceled, aborted, etc.: RobotWebTools/rosbridge_suite#953

Also @gonzalocasas Would you be able to review this PR?