Add sovereign cloud support for Microsoft Graph Provider

[dpmerron-ltd]

Summary

Adds provider-level sovereign cloud support via a new environment argument.

Users can now configure Azure China / 21Vianet with:

Closes #93

provider "msgraph" {
  environment = "china"
}

This change:

• Adds environment provider configuration with public as the default
• Supports public, china, and usgovernment
• Uses Azure SDK cloud configuration for authentication authority selection
• Requests Microsoft Graph tokens using the selected cloud’s Graph scope
• Sends Graph API requests to the selected cloud’s Graph endpoint
• Documents the new argument and Azure China example

Testing

• gofmt
• go test ./...
• make fmtcheck
• make test

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