Support AWS IoT

[happyRip]

Summary

Closes #143

Changes

Main:

• Add awsiot source

Other:

• Rename all source command variables to Command.
  • e.g. awsiot.Command instead of awsiot.AWSIoTCmd
• Define util.Unmarshaller interface that was not defined previously.

Testing

Tested by migrating a device without session from AWSIoT to TTS.

Need to test migration with session and an actual device.

Regressions

...

Notes for Reviewers

At the moment there are docstrings, readme and changelog entry missing. They will be added but the code can be reviewed at this point without them.

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Documentation: Relevant documentation is added or updated.
• Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.