Add DocumentDB support

[muaydin]

Problem

Users should be able to utilize DocumentDB features in the Explorer tab, it should list all the clusters available.
This will make DocumentDB more visible to developers.

Solution

Users can now view, create, and delete regional clusters and instances. Feedback is welcome. Work is still in progress.

View clusters and instances

• Create a new cluster

• Add an instance to a regional cluster

• Rename clusters & instances
• Start & stop a regional cluster
• Delete a cluster
• Delete an instance

• Modify an instance class

• Reboot an instance

• Polling mechanism to update when instances are in pending states (specifically 'creating', 'modifying', 'rebooting', 'starting', 'stopping', 'renaming')

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• Treat all work as PUBLIC. Private feature/x branches will not be squash-merged at release time.
• Your code changes must meet the guidelines in CONTRIBUTING.md.

License: I confirm that my contribution is made under the terms of the Apache 2.0 license.