Add configurable SignatureFieldParams class

Introduce `SignatureFieldParams` class to manage signature field dimensions with configurable `Width` and `Height`. Integrate this configuration into the service collection and replace hardcoded dimensions in `CreateEnvelopeCommandHandler.cs` and `SenderEditorPage.razor`. Update `appsettings.json` to include the new configuration section, allowing runtime adjustments.

EnvelopeGenerator.Application/Common/Configurations/SignatureFieldParams.cs

@@ -0,0 +1,17 @@
+namespace EnvelopeGenerator.Application.Common.Configurations;
+
+/// <summary>
+/// 
+/// </summary>
+public class SignatureFieldParams
+{
+    /// <summary>
+    /// 
+    /// </summary>
+    public double Width { get; init; } = 1.77;
+
+    /// <summary>
+    /// 
+    /// </summary>
+    public double Height { get; init; } = 1.96;
+}