Add support for custom cloud providers

[cheenamalhotra]

It maybe useful to allow user to provide settings for their cloud provider (one or multiple) when authenticating with AAD, by accepting below info:

• Cloud Name, e.g. "Azure Custom Cloud"
• Host URL: e.g. 'https://login.microsoftonline.com/',
• Microsoft resource endpoint: e.g. 'https://management.core.windows.net/',
• Graph resource endpoint: e.g. 'https://graph.microsoft.com/',
• ARM resource endpoint: e.g. 'https://management.azure.com/',
• and so on.. for all resources.

The cloud owners can authorize ADS' app ID as needed and enable authentication support on their end, to bring experience similar to SSMS:

Proposed solution:
New setting is introduced that accepts JSON object array for provider settings and if that's provided, the clouds gets appended to the list of clouds supports by ADS.

[Charles-Gagnon]

While you're at it, refactoring the cloud settings to not be checkboxes for enabling each cloud would be fantastic. Choosing the current cloud from a list makes a lot more sense given that we only allow one at a time anyways.

[cheenamalhotra]

We do allow and work with multiple clouds btw. Users get to choose which cloud to use for adding account from selector.

[Charles-Gagnon]

Oh neat, somehow I missed that. Carry on then!