Add AnnotationDto and AnnotationService for API integration

Added `AnnotationDto` to represent signature annotation positions,
including properties for ID, page, and coordinates, with detailed
documentation on coordinate systems. Implemented `AnnotationService`
to fetch annotation data from an API endpoint, using `HttpClient`
and supporting cancellation tokens. Registered `AnnotationService`
in the DI container to enable its use across the application.
Provided development setup details for working with fake data
via YARP-proxied routes.

EnvelopeGenerator.ReceiverUI/Program.cs

@@ -16,6 +16,7 @@ builder.Services.Configure<ApiOptions>(opts =>
 builder.Configuration.GetSection(ApiOptions.SectionName).Bind(opts));
 builder.Services.AddScoped<EnvelopeGenerator.ReceiverUI.Services.DocumentService>();
 builder.Services.AddScoped<EnvelopeGenerator.ReceiverUI.Services.AuthService>();
+builder.Services.AddScoped<EnvelopeGenerator.ReceiverUI.Services.AnnotationService>();
 
 builder.Services.AddDevExpressWebAssemblyBlazorReportViewer();
 builder.Services.AddDevExpressWebAssemblyBlazorPdfViewer();