chore: move Installation and Push to platform's domain folders (#5085)

* chore: set up a `CODEOWNERS` space for platform

* chore: move sql objects for `Installation` to platform's domain

* chore: move `Installation` and `PushRelay` code to platform's domain

src/Api/Platform/Installations/Models/InstallationRequestModel.cs

@@ -0,0 +1,23 @@
+using System.ComponentModel.DataAnnotations;
+using Bit.Core.Platform.Installations;
+using Bit.Core.Utilities;
+
+namespace Bit.Api.Platform.Installations;
+
+public class InstallationRequestModel
+{
+    [Required]
+    [EmailAddress]
+    [StringLength(256)]
+    public string Email { get; set; }
+
+    public Installation ToInstallation()
+    {
+        return new Installation
+        {
+            Key = CoreHelpers.SecureRandomString(20),
+            Email = Email,
+            Enabled = true
+        };
+    }
+}