cmd: add ability for providers to expose custom cmds

Add the ability for host providers to register themselves as offering
custom commands, under the provider ID name.

For example a provider with the ID 'foo' would be able to expose
commands under the `git-credential-manager-core foo <..>` command.

src/shared/Microsoft.Git.CredentialManager/Application.cs

@@ -17,6 +17,7 @@ public class Application : ApplicationBase, IConfigurableComponent
         private readonly string _appPath;
         private readonly IHostProviderRegistry _providerRegistry;
         private readonly IConfigurationService _configurationService;
+        private readonly IList<ProviderCommand> _providerCommands = new List<ProviderCommand>();
 
         public Application(ICommandContext context, string appPath)
             : this(context, new HostProviderRegistry(context), new ConfigurationService(context), appPath)
@@ -49,6 +50,13 @@ public void RegisterProvider(IHostProvider provider, HostProviderPriority priori
             {
                 _configurationService.AddComponent(configurableProvider);
             }
+
+            // If the provider has custom commands to offer then create them here
+            if (provider is ICommandProvider cmdProvider)
+            {
+                ProviderCommand providerCommand = cmdProvider.CreateCommand();
+                _providerCommands.Add(providerCommand);
+            }
         }
 
         protected override async Task<int> RunInternalAsync(string[] args)
@@ -62,6 +70,11 @@ protected override async Task<int> RunInternalAsync(string[] args)
             rootCommand.AddCommand(new ConfigureCommand(Context, _configurationService));
             rootCommand.AddCommand(new UnconfigureCommand(Context, _configurationService));
 
+            // Add any custom provider commands
+            foreach (ProviderCommand providerCommand in _providerCommands)
+            {
+                rootCommand.AddCommand(providerCommand);
+            }
 
             // Trace the current version and program arguments
             Context.Trace.WriteLine($"{Constants.GetProgramHeader()} '{string.Join(" ", args)}'");

src/shared/Microsoft.Git.CredentialManager/Commands/ProviderCommand.cs

@@ -0,0 +1,20 @@
+using System.CommandLine;
+
+namespace Microsoft.Git.CredentialManager.Commands
+{
+    public interface ICommandProvider
+    {
+        /// <summary>
+        /// Create a custom provider command.
+        /// </summary>
+        ProviderCommand CreateCommand();
+    }
+
+    public class ProviderCommand : Command
+    {
+        public ProviderCommand(IHostProvider provider)
+            : base(provider.Id, $"Commands for interacting with the {provider.Name} host provider")
+        {
+        }
+    }
+}