WebSub webhook implementations should respond to the event notification request before running the agent

[MaryamZi]

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See https://www.w3.org/TR/websub/#content-distribution

Subscribers SHOULD respond to content distribution requests as quickly as possible; their success response code SHOULD only indicate receipt of the message, not acknowledgment that it was successfully processed by the subscriber

There seems to be an open issue for the Ballerina WebSub library - ballerina-platform/ballerina-library#3022

We can fix it at least for the Python implementation for now.