Add ApiOptions and configure API settings

Added a new `ApiOptions` class to encapsulate API configuration,
including the `BaseUrl` property and `SectionName` constant.
Registered `ApiOptions` in the dependency injection container
in `Program.cs` and bound it to the "Api" section in the
configuration file. Updated `appsettings.json` to include the
"Api" section with a `BaseUrl` value of "https://localhost:8088".

EnvelopeGenerator.ReceiverUI/Options/ApiOptions.cs

@@ -0,0 +1,8 @@
+namespace EnvelopeGenerator.ReceiverUI.Options;
+
+public class ApiOptions
+{
+    public const string SectionName = "Api";
+
+    public string BaseUrl { get; set; } = string.Empty;
+}