Add user registration emails + email infrastructure (#150)

* Add email functionality

* Add email template project and rendering service

* Update email template dir structure

* Add email header and footer components for user registration template

* update example env

* Refactor email templates namespace and components

* Format email dir

src/Core/Infrastructure/Infrastructure.Email.Templates/Rendering/IEmailTemplateProvider.cs

@@ -0,0 +1,18 @@
+namespace Infrastructure.Email.Templates.Rendering;
+
+/// <summary>
+/// Service for rendering Razor email templates to HTML.
+/// </summary>
+public interface IEmailTemplateProvider
+{
+    /// <summary>
+    /// Renders the UserRegisteredEmail template with the specified parameters.
+    /// </summary>
+    /// <param name="username">The username to include in the email</param>
+    /// <param name="confirmationLink">The email confirmation link</param>
+    /// <returns>The rendered HTML string</returns>
+    Task<string> RenderUserRegisteredEmailAsync(
+        string username,
+        string confirmationLink
+    );
+}