Add cloudhook support to SmartThings component

[andrewsayre]

Description:

Adds cloudhook support through Home Assistant Cloud (Nabu Casa) to the SmartThings component. Highlights:

• The default behavior for new integrations will be to use a cloudhook via Home Assistant Cloud (when logged in with a non-expired subscription) to receive push updates from the SmartThings cloud. This eliminates the need to expose the local Home Assistant to the internet and greatly simplifies setup.
• Existing configured integrations will continue to function using the local webhook and must be removed in order to activate the new feature.

Pull request in home-assistant.io with documentation: home-assistant/home-assistant.io#8902

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• New dependencies have been added to the REQUIREMENTS variable (example).
• New dependencies are only imported inside functions that use them (example).
• New or updated dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.

If the code does not interact with devices:

• Tests have been added to verify that the new code works.