feat: Referenztypen zwischen Backend und Frontend standardisieren

- `GetReferenceTypes`-Endpunkt im `HistoryController` hinzugefügt, um Key-Value-Paare des `ReferenceType`-Enums im camelCase-Format bereitzustellen.
- `ConfigurationService` in Angular erweitert, um die `ReferenceType`-Werte vom neuen API-Endpunkt abzurufen und zu speichern.

EnvelopeGenerator.GeneratorAPI/Controllers/HistoryController.cs

@@ -21,7 +21,25 @@ namespace EnvelopeGenerator.GeneratorAPI.Controllers
             _service = service;
         }
 
+        [HttpGet("reference-type")]
+        [Authorize]
+        public IActionResult GetReferenceTypes()
+        {
+            // Enum to Key-Value pair
+            var referenceTypes = Enum.GetValues(typeof(ReferenceType))
+                                     .Cast<ReferenceType>()
+                                     .ToDictionary(rt =>
+                                     {
+                                         var key = rt.ToString();
+                                         var keyAsCamelCase = char.ToLower(key[0]) + key[1..];
+                                         return keyAsCamelCase;
+                                     }, rt => (int)rt);
+
+            return Ok(referenceTypes);
+        }
+
         [HttpGet]
+        [Authorize]
         public async Task<IActionResult> GetAllAsync([FromQuery] int? envelopeId = null, [FromQuery] string? userReference = null, [FromQuery] int? referenceType = null, [FromQuery] int? status = null, [FromQuery] bool withSender = false, [FromQuery] bool withReceiver = false)
         {
             ReferenceType? refTypEnum = null;