[PM-19659] Clean up Notifications code (#6244)

* Move PushType to Platform Folder

- Move the PushType next to the rest of push notification code
- Specifically exclude it from needing Platform code review
- Add tests establishing rules Platform has for usage of this enum, making it safe to have no owner

* Move NotificationHub code into Platform/Push directory

* Update NotificationHub namespace imports

* Add attribute for storing push type metadata

* Rename Push Engines to have PushEngine suffix

* Move Push Registration items to their own directory

* Push code move

* Add expected usage comment

* Add Push feature registration method

- Make method able to be called multipes times with no ill effects

* Add Push Registration service entrypoint and tests

* Use new service entrypoints

* Test changes

test/Core.Test/Platform/PushRegistration/PushRegistrationServiceCollectionExtensionsTests.cs

@@ -0,0 +1,108 @@
+using Bit.Core.Platform.Push;
+using Bit.Core.Platform.PushRegistration.Internal;
+using Bit.Core.Repositories;
+using Bit.Core.Repositories.Noop;
+using Bit.Core.Settings;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.DependencyInjection.Extensions;
+using Xunit;
+
+namespace Bit.Core.Test.Platform.PushRegistration;
+
+public class PushRegistrationServiceCollectionExtensionsTests
+{
+    [Fact]
+    public void AddPushRegistration_Cloud_CreatesNotificationHubRegistrationService()
+    {
+        var services = Build(new Dictionary<string, string?>
+        {
+            { "GlobalSettings:SelfHosted", "false" },
+        });
+
+        var pushRegistrationService = services.GetRequiredService<IPushRegistrationService>();
+        Assert.IsType<NotificationHubPushRegistrationService>(pushRegistrationService);
+    }
+
+    [Fact]
+    public void AddPushRegistration_SelfHosted_NoOtherConfig_ReturnsNoopRegistrationService()
+    {
+        var services = Build(new Dictionary<string, string?>
+        {
+            { "GlobalSettings:SelfHosted", "true" },
+        });
+
+        var pushRegistrationService = services.GetRequiredService<IPushRegistrationService>();
+        Assert.IsType<NoopPushRegistrationService>(pushRegistrationService);
+    }
+
+    [Fact]
+    public void AddPushRegistration_SelfHosted_RelayConfig_ReturnsRelayRegistrationService()
+    {
+        var services = Build(new Dictionary<string, string?>
+        {
+            { "GlobalSettings:SelfHosted", "true" },
+            { "GlobalSettings:PushRelayBaseUri", "https://example.com" },
+            { "GlobalSettings:Installation:Key", "some_key" },
+        });
+
+        var pushRegistrationService = services.GetRequiredService<IPushRegistrationService>();
+        Assert.IsType<RelayPushRegistrationService>(pushRegistrationService);
+    }
+
+    [Fact]
+    public void AddPushRegistration_MultipleTimes_NoAdditionalServices()
+    {
+        var services = new ServiceCollection();
+
+        var config = new Dictionary<string, string?>
+        {
+            { "GlobalSettings:SelfHosted", "true" },
+            { "GlobalSettings:PushRelayBaseUri", "https://example.com" },
+            { "GlobalSettings:Installation:Key", "some_key" },
+        };
+
+        AddServices(services, config);
+
+        // Add services again
+        services.AddPushRegistration();
+
+        var provider = services.BuildServiceProvider();
+
+        Assert.Single(provider.GetServices<IPushRegistrationService>());
+    }
+
+    private static ServiceProvider Build(Dictionary<string, string?> initialData)
+    {
+        var services = new ServiceCollection();
+
+        AddServices(services, initialData);
+
+        return services.BuildServiceProvider();
+    }
+
+    private static void AddServices(IServiceCollection services, Dictionary<string, string?> initialData)
+    {
+        // A minimal service collection is always expected to have logging, config, and global settings
+        // pre-registered. 
+
+        services.AddLogging();
+
+        var config = new ConfigurationBuilder()
+            .AddInMemoryCollection(initialData)
+            .Build();
+
+        services.TryAddSingleton(config);
+        var globalSettings = new GlobalSettings();
+        config.GetSection("GlobalSettings").Bind(globalSettings);
+
+        services.TryAddSingleton(globalSettings);
+        services.TryAddSingleton<IGlobalSettings>(globalSettings);
+
+
+        // Temporary until AddPushRegistration can add it themselves directly.
+        services.TryAddSingleton<IInstallationDeviceRepository, InstallationDeviceRepository>();
+
+        services.AddPushRegistration();
+    }
+}