Greenfield API: Get current User

Builds on #1368
This PR adds a new endpoint: Get current user.. It only returns the current user's id and email for now( let's extend later)
It also adds a new permission: `ProfileManagement` which is needed for this endpoint (and for update endpoints later)

BTCPayServer.Client/Models/ApplicationUserData.cs

@@ -0,0 +1,8 @@
+namespace BTCPayServer.Client.Models
+{
+    public class ApplicationUserData
+    {
+        public string Id { get; set; }
+        public string Email { get; set; }
+    }
+}