[PM-23493] Generic mailer proposal (#5958)

This implements a new Mailer service which supersedes the `HandlebarsMailService`. It allows teams to create emails without having to extend a generic service.

The `IMailer` only contains a single method, `SendEmail`, which sends an instance of `BaseMail`.

test/Core.Test/Platform/Mailer/HandlebarMailRendererTests.cs

@@ -0,0 +1,20 @@
+using Bit.Core.Platform.Mailer;
+using Bit.Core.Test.Platform.Mailer.TestMail;
+using Xunit;
+
+namespace Bit.Core.Test.Platform.Mailer;
+
+public class HandlebarMailRendererTests
+{
+    [Fact]
+    public async Task RenderAsync_ReturnsExpectedHtmlAndTxt()
+    {
+        var renderer = new HandlebarMailRenderer();
+        var view = new TestMailView { Name = "John Smith" };
+
+        var (html, txt) = await renderer.RenderAsync(view);
+
+        Assert.Equal("Hello <b>John Smith</b>", html.Trim());
+        Assert.Equal("Hello John Smith", txt.Trim());
+    }
+}