Add minimal support for ROS2

[gonzalocasas]

This PR builds up on https://github.com/gramaziokohler/roslibpy/pull/120 but makes it a non-breaking change instead. Added ROS2 to the CI tests also, although only testing one tiny part of ROS2 so far.

What type of change is this?

• [ ] Bug fix in a backwards-compatible manner.
• [x] 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

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• [x] I added a line to the CHANGELOG.rst file in the Unreleased section under the most fitting heading (e.g. Added, Changed, Removed).
• [x] 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).
• [x] I have added tests that prove my fix is effective or that my feature works.
• [x] I have added necessary documentation (if appropriate)

[gonzalocasas]

@iliabaranov I've moved your fix to a roslibpy.ros2.Header class and now everything seems to work on both ROS1 and ROS2. I will merge and cut a new release with this.

[iliabaranov]

Cheers, thanks! :D

[gonzalocasas]

I'll try to add actionlib support soon and fix the rest of the tests

[gonzalocasas]

Please let me know if you encounter any issues in the meantime