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/PdfSigning/SignPdfByElementCommandValidator.cs

@@ -0,0 +1,18 @@
+using FluentValidation;
+
+namespace EnvelopeGenerator.Application.PdfSigning;
+
+public class SignPdfByElementCommandValidator : AbstractValidator<SignPdfByElementCommand>
+{
+    public SignPdfByElementCommandValidator()
+    {
+        RuleFor(x => x)
+            .Must(x => (x.Element is not null) ^ x.ElementId.HasValue)
+            .WithMessage("Exactly one of Element or ElementId must be provided.");
+
+        RuleFor(x => x.ElementId)
+            .GreaterThan(0)
+            .When(x => x.ElementId.HasValue)
+            .WithMessage("ElementId must be greater than 0.");
+    }
+}