Add support for Azure Government Cloud (GovCloud) environments

[Copilot]

This PR adds comprehensive support for Azure Government Cloud environments to Azure Key Vault Explorer, allowing users to connect to both Azure Commercial and Azure US Government clouds.

Changes Made

Cloud Environment Configuration

• Added CloudEnvironment class with predefined configurations for Public and US Government clouds
• Created AzureCloud enum to represent different cloud types
• Added cloud selection setting to AppSettings with validation

Authentication Updates

• Modified AuthService to use cloud-specific authorities:
  • Public: login.microsoftonline.com
  • US Government: login.microsoftonline.us
• Updated token acquisition methods to use correct API scopes for each cloud:
  • Public: vault.azure.net, management.core.windows.net
  • US Government: vault.usgovcloudapi.net, management.usgovcloudapi.net

User Interface

• Added cloud selection dropdown in Settings page with descriptive text
• Integrated with existing settings infrastructure for persistence
• Cloud selection is saved and remembered between app restarts

Portal Integration

• Updated "Open in Azure" functionality to use correct portal URLs:
  • Public: portal.azure.com
  • US Government: portal.azure.us

Key Features

Multi-Cloud Support: Users can now select between Azure Public Cloud and Azure US Government from a dropdown in Settings.

Automatic Configuration: All endpoints (authentication, Key Vault API, Resource Manager API, and portal URLs) are automatically configured based on the selected cloud environment.

Backwards Compatibility: Existing installations continue to use Azure Public Cloud by default with no changes required.

Settings Persistence: Cloud selection is saved to user settings and persists between application restarts.

Testing

Created comprehensive tests validating:

• Cloud environment endpoint configuration
• Enum parsing with fallback handling for invalid values
• Constants helper methods return correct scopes
• All US Government endpoints match Azure documentation

Usage

1. Navigate to Settings page
2. Select desired cloud from "Azure Cloud Environment" dropdown
3. Sign out and sign back in to authenticate against the new cloud
4. Access Key Vault resources in the selected cloud environment

This implementation provides a complete solution for organizations requiring Azure Government Cloud access while maintaining full compatibility for existing commercial Azure users.

Fixes #122.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• av-build-tel-api-v1.avaloniaui.net
  • Triggering command: dotnet exec --runtimeconfig /home/REDACTED/.nuget/packages/avalonia.buildservices/0.0.31/tools/netstandard2.0/runtimeconfig.json /home/REDACTED/.nuget/packages/avalonia.buildservices/0.0.31/tools/netstandard2.0/Avalonia.BuildServices.Collector.dll (dns block)
  • Triggering command: dotnet build --verbosity minimal (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[Copilot]

@cricketthomas 👋 This repository doesn't have Copilot instructions. With Copilot instructions, I can understand the repository better, work faster and produce higher quality PRs.

I can generate a .github/copilot-instructions.md file for you automatically. Click here to open a pre-filled issue and assign it to me. I'll write the instructions, and then tag you for review.

[cricketthomas]

Fails on launch with:
System.ArgumentException: 'The authority URI should have at least one segment in the path (i.e. https://<host>/<path>/...). (Parameter 'authority')'