feat: Add termination functionality for rental agreements

- Introduced new entity `CKuendigung` for managing terminations.
- Added fields for termination details including date, reason, type, and status.
- Implemented backend service to initiate terminations from rental agreements.
- Created frontend handler for termination actions with confirmation dialog.
- Updated metadata and layouts for `CKuendigung` to support new functionality.
- Added internationalization support for English and German languages.
- Enhanced existing entities to establish relationships with terminations.

custom/Espo/Custom/Resources/metadata/entityDefs/CVmhMietverhltnis.json

@@ -280,6 +280,14 @@
             "entity": "CMietobjekt",
             "audited": false,
             "isCustom": true
+        },
+        "kuendigungen": {
+            "type": "hasMany",
+            "relationName": "cKuendigungVmhMietverhltnis",
+            "foreign": "vmhMietverhltnis",
+            "entity": "CKuendigung",
+            "audited": false,
+            "isCustom": true
         }
     },
     "collection": {