Update ReceiverController namespace and enhance Get method

The namespace of the `ReceiverController` class was updated from `EnvelopeGenerator.GeneratorAPI.Controllers` to `EnvelopeGenerator.Server.Controllers`.

The `Get` method was enhanced with the following changes:
- Added `[Authorize]` attribute with `AuthScheme.Sender`.
- Updated the method signature to include an optional `ReadReceiverQuery? receiver` parameter (defaulting to `null`) and a new `bool onlyEmailAddress` parameter (defaulting to `false`).
- Modified logic to handle `receiver` being `null` by creating a new `ReadReceiverQuery`.
- Added handling for `onlyEmailAddress` to return a list of email addresses if true.
- Simplified the result handling and removed the previous `receiver.HasAnyCriteria` logic.

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Controllers/ReceiverController.cs

@@ -3,7 +3,7 @@ using EnvelopeGenerator.Application.Receivers.Queries;
 using Microsoft.AspNetCore.Authorization;
 using Microsoft.AspNetCore.Mvc;
 
-namespace EnvelopeGenerator.GeneratorAPI.Controllers;
+namespace EnvelopeGenerator.Server.Controllers;
 
 /// <summary>
 /// Controller für die Verwaltung von Empfängern.
@@ -33,15 +33,16 @@ public class ReceiverController : ControllerBase
     /// <param name="receiver">Die Abfrageparameter, einschließlich E-Mail-Adresse und Signatur.</param>
     /// <returns>Eine Liste von Empfängern oder ein Fehlerstatus.</returns>
     [HttpGet]
-    public async Task<IActionResult> Get([FromQuery] ReadReceiverQuery receiver)
+    [Authorize(AuthenticationSchemes = AuthScheme.Sender)]
+    public async Task<IActionResult> Get([FromQuery] ReadReceiverQuery? receiver = null, [FromQuery] bool onlyEmailAddress = false)
     {
-        if (!receiver.HasAnyCriteria)
-        {
-            var all = await _mediator.Send(new ReadReceiverQuery());
-            return Ok(all);
-        }
+        var result = await _mediator.Send(receiver ?? new ReadReceiverQuery());
 
-        var result = await _mediator.Send(receiver);
-        return result is null ? NotFound() : Ok(result);
+        if (result is null)
+            return NotFound();
+        else if (onlyEmailAddress)
+            return Ok(result.Select(r => r.EmailAddress).ToList());
+        else
+            return Ok(result);
     }
 }