Add IRequestContext interface and RequestContext class

Introduce the IRequestContext interface with an Origin property
in the EnvelopeGenerator.Application.Common.Interfaces namespace.
Implement this interface in the new RequestContext class within
EnvelopeGenerator.Server.Services, using IHttpContextAccessor to
access the Origin header. Register IRequestContext with the DI
container in Program.cs using AddScoped. Update using directives
to include necessary namespaces and reorganize for clarity.

EnvelopeGenerator.Application/Common/Interfaces/IRequestContext.cs

@@ -0,0 +1,12 @@
+namespace EnvelopeGenerator.Application.Common.Interfaces;
+
+/// <summary>
+/// 
+/// </summary>
+public interface IRequestContext
+{
+    /// <summary>
+    /// 
+    /// </summary>
+    string? Origin { get; }
+}