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/wwwroot/appsettings.json

@@ -0,0 +1,5 @@
+{
+  "Api": {
+    "BaseUrl": "https://localhost:8088"
+  }
+}