Add YARP reverse proxy route for auth login requests

Configured yarp.json to proxy POST /api/auth requests to the
auth-hub cluster at http://172.24.12.39:9090, rewriting the
path to /api/auth/sign-flow before forwarding.

EnvelopeGenerator.API/yarp.json

@@ -0,0 +1,25 @@
+{
+  "ReverseProxy": {
+    "Routes": {
+      "auth-login": {
+        "ClusterId": "auth-hub",
+        "Match": {
+          "Path": "/api/auth",
+          "Methods": [ "POST" ]
+        },
+        "Transforms": [
+          { "PathSet": "/api/auth/sign-flow" }
+        ]
+      }
+    },
+    "Clusters": {
+      "auth-hub": {
+        "Destinations": {
+          "primary": {
+            "Address": "http://172.24.12.39:9090"
+          }
+        }
+      }
+    }
+  }
+}