[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

src/Identity/Utilities/ServiceCollectionExtensions.cs

@@ -1,6 +1,7 @@
 using Bit.Core.Auth.Repositories;
 using Bit.Core.IdentityServer;
 using Bit.Core.Settings;
+using Bit.Core.Tools.Models.Data;
 using Bit.Core.Utilities;
 using Bit.Identity.IdentityServer;
 using Bit.Identity.IdentityServer.ClientProviders;
@@ -26,7 +27,8 @@ public static class ServiceCollectionExtensions
         services.AddTransient<IDeviceValidator, DeviceValidator>();
         services.AddTransient<ITwoFactorAuthenticationValidator, TwoFactorAuthenticationValidator>();
         services.AddTransient<ILoginApprovingClientTypes, LoginApprovingClientTypes>();
-        services.AddTransient<ISendPasswordRequestValidator, SendPasswordRequestValidator>();
+        services.AddTransient<ISendAuthenticationMethodValidator<ResourcePassword>, SendPasswordRequestValidator>();
+        services.AddTransient<ISendAuthenticationMethodValidator<EmailOtp>, SendEmailOtpRequestValidator>();
 
         var issuerUri = new Uri(globalSettings.BaseServiceUri.InternalIdentity);
         var identityServerBuilder = services