Add support for signing PDFs with DocReceiverElement

Extended `DocReceiverElementDto` to include an optional `Document` reference. Enhanced `ReadDocReceiverElementQuery` with `Id` and `IncludeDocument` properties for filtering and document inclusion.

Updated `ReadDocReceiverElementQueryHandler` to handle new query properties, enabling filtering by `Id` and optionally including associated documents.

Introduced `SignPdfByElementCommand` and its handler to support signing PDFs using either a provided `DocReceiverElementDto` or an `ElementId`. Added validation via `SignPdfByElementCommandValidator` to ensure proper usage.

Refactored and added necessary `using` directives for new dependencies.

EnvelopeGenerator.Application/DocReceiverElements/Queries/ReadDocReceiverElementQuery.cs

@@ -14,6 +14,15 @@ namespace EnvelopeGenerator.Application.DocReceiverElements.Queries;
 /// </summary>
 public record ReadDocReceiverElementQuery : EnvelopeReceiverQueryBase, IRequest<IEnumerable<DocReceiverElementDto>>
 {
+    /// <summary>
+    /// 
+    /// </summary>
+    public int? Id { get; set; } = null;
+
+    /// <summary>
+    /// 
+    /// </summary>
+    public bool IncludeDocument { get; set; } = false;
 }
 
 /// <summary>
@@ -47,7 +56,10 @@ public class ReadDocReceiverElementQueryHandler : IRequestHandler<ReadDocReceive
     {
         var q = _repository.Query;
 
-        if(request.Envelope.Id is int envelopeId)
+        if (request.Id is not null)
+            q = q.Where(e => e.Id == request.Id);
+
+        if (request.Envelope.Id is int envelopeId)
             q = q.Where(e => e.Document.EnvelopeId == envelopeId);
 
         if (request.Envelope.Uuid is string envelopeUuid)
@@ -59,6 +71,9 @@ public class ReadDocReceiverElementQueryHandler : IRequestHandler<ReadDocReceive
         if (request.Receiver.Signature is string signature)
             q = q.Where(e => e.Receiver.Signature == signature);
 
+        if( request.IncludeDocument)
+            q = q.Include(e => e.Document);
+
         var elements = await q.ToListAsync(cancellationToken);
 
         return _mapper.Map<IEnumerable<DocReceiverElementDto>>(elements);