Add ReadDocReceiverElementQuery and handler

Introduced `ReadDocReceiverElementQuery` for retrieving document receiver elements and its corresponding handler. Added necessary `using` directives for dependencies like `AutoMapper`, `MediatR`, and `Microsoft.EntityFrameworkCore`.

The handler dynamically filters `DocReceiverElement` data based on optional query parameters (e.g., `Envelope.Id`, `Envelope.Uuid`, `Receiver.Id`, `Receiver.Signature`) using LINQ. Data is fetched asynchronously and mapped to DTOs using `AutoMapper`.

EnvelopeGenerator.Application/DocReceiverElements/Queries/ReadDocReceiverElementQuery.cs

@@ -0,0 +1,66 @@
+using AutoMapper;
+using EnvelopeGenerator.Application.Common.Dto;
+using EnvelopeGenerator.Application.Common.Query;
+using EnvelopeGenerator.Domain.Entities;
+using MediatR;
+using EnvelopeGenerator.Application.Common.Extensions;
+using DigitalData.Core.Abstraction.Application.Repository;
+using Microsoft.EntityFrameworkCore;
+
+namespace EnvelopeGenerator.Application.DocReceiverElements.Queries;
+
+/// <summary>
+/// 
+/// </summary>
+public record ReadDocReceiverElementQuery : EnvelopeReceiverQueryBase, IRequest<IEnumerable<DocReceiverElementDto>>
+{
+}
+
+/// <summary>
+/// 
+/// </summary>
+public class ReadDocReceiverElementQueryHandler : IRequestHandler<ReadDocReceiverElementQuery, IEnumerable<DocReceiverElementDto>>
+{
+    private readonly IRepository<DocReceiverElement> _repository;
+
+    private readonly IMapper _mapper;
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="repository"></param>
+    /// <param name="mapper"></param>
+    public ReadDocReceiverElementQueryHandler(IRepository<DocReceiverElement> repository, IMapper mapper)
+    {
+        _repository = repository;
+        _mapper = mapper;
+    }
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="request"></param>
+    /// <param name="cancellationToken"></param>
+    /// <returns></returns>
+    /// <exception cref="NotImplementedException"></exception>
+    public async Task<IEnumerable<DocReceiverElementDto>> Handle(ReadDocReceiverElementQuery request, CancellationToken cancellationToken)
+    {
+        var q = _repository.Query;
+
+        if(request.Envelope.Id is int envelopeId)
+            q = q.Where(e => e.Document.EnvelopeId == envelopeId);
+
+        if (request.Envelope.Uuid is string envelopeUuid)
+            q = q.Where(e => e.Document.Envelope.Uuid == envelopeUuid);
+
+        if (request.Receiver.Id is int receiverId)
+            q = q.Where(e => e.ReceiverId == receiverId);
+
+        if (request.Receiver.Signature is string signature)
+            q = q.Where(e => e.Receiver.Signature == signature);
+
+        var elements = await q.ToListAsync(cancellationToken);
+
+        return _mapper.Map<IEnumerable<DocReceiverElementDto>>(elements);
+    }
+}