[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/MailerTest.cs

@@ -0,0 +1,37 @@
+using Bit.Core.Models.Mail;
+using Bit.Core.Platform.Mailer;
+using Bit.Core.Services;
+using Bit.Core.Test.Platform.Mailer.TestMail;
+using NSubstitute;
+using Xunit;
+
+namespace Bit.Core.Test.Platform.Mailer;
+
+public class MailerTest
+{
+    [Fact]
+    public async Task SendEmailAsync()
+    {
+        var deliveryService = Substitute.For<IMailDeliveryService>();
+        var mailer = new Core.Platform.Mailer.Mailer(new HandlebarMailRenderer(), deliveryService);
+
+        var mail = new TestMail.TestMail()
+        {
+            ToEmails = ["test@bw.com"],
+            View = new TestMailView() { Name = "John Smith" }
+        };
+
+        MailMessage? sentMessage = null;
+        await deliveryService.SendEmailAsync(Arg.Do<MailMessage>(message =>
+            sentMessage = message
+        ));
+
+        await mailer.SendEmail(mail);
+
+        Assert.NotNull(sentMessage);
+        Assert.Contains("test@bw.com", sentMessage.ToEmails);
+        Assert.Equal("Test Email", sentMessage.Subject);
+        Assert.Equivalent("Hello John Smith", sentMessage.TextContent.Trim());
+        Assert.Equivalent("Hello <b>John Smith</b>", sentMessage.HtmlContent.Trim());
+    }
+}