Add ReceiverService and enhance DocumentService

Added `ReceiverService` to the dependency injection container in `Program.cs` and implemented its functionality for fetching receiver email suggestions via API.

Enhanced `DocumentService` with a new method `GetSenderDocumentDetailsByEnvelopeIdAsync` to fetch sender document details by envelope ID. Introduced JSON handling improvements with `JsonSerializerOptions`.

These changes improve support for receiver-related operations and extend document-related capabilities.

EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Program.cs

@@ -42,6 +42,7 @@ builder.Services.AddScoped<SignatureCacheService>();
 builder.Services.AddSingleton<AppVersionService>();
 builder.Services.AddScoped<EnvelopeService>();
 builder.Services.AddScoped<LocalizationService>();
+builder.Services.AddScoped<ReceiverService>();
 
 // UI Preferences & Theme
 builder.Services.AddScoped<UserPreferencesService>();