feat(sender): add envelope draft save command and endpoint

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Controllers/EnvelopeController.cs

@@ -123,4 +123,12 @@ public class EnvelopeController : ControllerBase
 
         return Ok(result);
     }
+
+    [Authorize(AuthenticationSchemes = AuthScheme.Sender)]
+    [HttpPost("draft")]
+    public async Task<IActionResult> SaveDraftAsync([FromBody] SaveEnvelopeDraftCommand command)
+    {
+        var result = await _mediator.Send(command.WithAuth(User.GetId()));
+        return Ok(result);
+    }
 }