fix: existing_requests in RequestSynchronizerResolver should be initialized (#158)

- Closes: #157

Author: Niraj-Kamdar

packages/polywrap-uri-resolvers/polywrap_uri_resolvers/resolvers/cache/request_synchronizer_resolver.py

@@ -64,6 +64,7 @@ def __init__(
             options (Optional[RequestSynchronizerResolverOptions]):\
                 The options to use.
         """
+        self.existing_requests = {}
         self.resolver_to_synchronize = resolver_to_synchronize
         self.options = options