Refactor email template commands and controller logic

- Updated namespace for `ResetEmailTemplateCommand` and added a constructor for flexible initialization.
- Introduced `ChangedWhen` property in `UpdateEmailTemplateCommand` to track modification time.
- Refactored `UpdateEmailTemplateCommandHandler` for improved email template retrieval logic.
- Modified `EmailTemplateController` method signatures and logic for clarity and consistency.
- Added `EmailTemplate` entry in `appsettings.json` for database trigger configuration.

EnvelopeGenerator.GeneratorAPI/appsettings.json

@@ -169,7 +169,8 @@
     "EnvelopeHistory": [ "TBSIG_ENVELOPE_HISTORY_AFT_INS" ],
     "EmailOut": [ "TBEMLP_EMAIL_OUT_AFT_INS", "TBEMLP_EMAIL_OUT_AFT_UPD" ],
     "EnvelopeReceiverReadOnly": [ "TBSIG_ENVELOPE_RECEIVER_READ_ONLY_UPD" ],
-    "Receiver": []
+    "Receiver": [],
+    "EmailTemplate": [ "TBSIG_EMAIL_TEMPLATE_AFT_UPD" ]
   },
   "MainPageTitle": null,
   "AnnotationParams": {