Add API endpoint to retrieve envelope receiver by key

Added a new `GetEnvelopeReceiverOfReceiver` method in the
`EnvelopeReceiverController` to retrieve an envelope receiver
based on the provided `envelopeKey`. The method is secured
with the `Receiver` authorization policy and uses the
`_mediator.Send` method to process a `ReadEnvelopeReceiverQuery`.

Included the `Microsoft.Extensions.Options` namespace to
support potential configuration needs. Added XML documentation
placeholders for the new method.

EnvelopeGenerator.API/Controllers/EnvelopeReceiverController.cs

@@ -14,6 +14,7 @@ using EnvelopeGenerator.Application.Common.SQL;
 using EnvelopeGenerator.Application.Common.Dto.Receiver;
 using EnvelopeGenerator.Application.Common.Interfaces.SQLExecutor;
 using EnvelopeGenerator.API.Extensions;
+using EnvelopeGenerator.Domain.Constants;
 
 namespace EnvelopeGenerator.API.Controllers;
 
@@ -73,6 +74,24 @@ public class EnvelopeReceiverController : ControllerBase
         return Ok(result);
     }
 
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="envelopeKey"></param>
+    /// <param name="cancel"></param>
+    /// <returns></returns>
+    [Authorize(Policy = AuthPolicy.Receiver)]
+    [HttpGet("{envelopeKey}")]
+    public async Task<IActionResult> GetEnvelopeReceiverOfReceiver([FromRoute] string envelopeKey, CancellationToken cancel)
+    {
+        var er = await _mediator.Send(new ReadEnvelopeReceiverQuery()
+        {
+            Key = envelopeKey
+        }, cancel);
+
+        return Ok(er);
+    }
+
     /// <summary>
     /// Ruft den Namen des zuletzt verwendeten Empfängers basierend auf der angegebenen E-Mail-Adresse ab.
     /// </summary>