Add Enpower Smart Switch status

[gtdiehl]

Adding the status of the Enpower Smart Switch will let the user know if their system is attached or detached from the grid. This is only supported for systems that have the Enpower Smart Switch installed.

Example output when the device is installed:

...
enpower_status:          {'connected': True, 'grid_status': 'closed'}

Example output when the device is not installed or running the API against an Envoy that does not support it:

...
enpower_status:          Enpower Smart Switch status is not available for your Envoy device.

Pull request recommendations:

• Name your pull request your-development-type/short-description. Ex: feature/read-tiff-files
• Link to any relevant issue in the PR description. Ex: Partially Issue Add Envoy data from Encharge batteries #72
• Provide context of changes.
• Provide relevant tests for your feature or bug fix.
• Provide or update documentation for any feature added by your pull request.

Thanks for contributing!