[PM-22678] Send email otp authentication method (#6255)

feat(auth): email OTP validation, and generalize authentication interface

- Generalized send authentication method interface
- Made validate method async
- Added email mail support for Handlebars
- Modified email templates to match future implementation

fix(auth): update constants, naming conventions, and error handling

- Renamed constants for clarity
- Updated claims naming convention
- Fixed error message generation
- Added customResponse for Rust consumption

test(auth): add and fix tests for validators and email

- Added tests for SendEmailOtpRequestValidator
- Updated tests for SendAccessGrantValidator

chore: apply dotnet formatting

test/Core.Test/Services/HandlebarsMailServiceTests.cs

@@ -247,11 +247,18 @@ public class HandlebarsMailServiceTests
         }
     }
 
-    // Remove this test when we add actual tests. It only proves that
-    // we've properly constructed the system under test.
     [Fact]
-    public void ServiceExists()
+    public async Task SendSendEmailOtpEmailAsync_SendsEmail()
     {
-        Assert.NotNull(_sut);
+        // Arrange
+        var email = "test@example.com";
+        var token = "aToken";
+        var subject = string.Format("Your Bitwarden Send verification code is {0}", token);
+
+        // Act
+        await _sut.SendSendEmailOtpEmailAsync(email, token, subject);
+
+        // Assert
+        await _mailDeliveryService.Received(1).SendEmailAsync(Arg.Any<MailMessage>());
     }
 }