Update signature handling and database connection management

- Changed `Signature` properties from `int` to `double` for precise positioning.
- Added necessary using directives in `EnvelopeReceiverController.cs` for database operations.
- Introduced a private `_cnnStr` field in `EnvelopeReceiverController` to store the connection string.
- Updated the constructor to accept `IOptions<ConnectionString>` for dependency injection of the connection string.
- Added a SQL command block to handle document element additions using a stored procedure.
- Configured the `ConnectionString` class in `Program.cs` to bind the connection string from app settings.
- Created a new `ConnectionString` class to manage the connection string value.

EnvelopeGenerator.GeneratorAPI/Models/ConnectionString.cs

@@ -0,0 +1,12 @@
+namespace EnvelopeGenerator.GeneratorAPI.Models;
+
+/// <summary>
+/// 
+/// </summary>
+public class ConnectionString
+{
+    /// <summary>
+    /// 
+    /// </summary>
+    public string Value { get; set; } = string.Empty;
+}