Using express.json to get the request body as JSON

[Eyal-Be]

Description

@dhatcher42 was trying to follow the Debugging Webhooks locally guide, and was getting an error during the validation of the webhook request step. The validation error occurs due to the body of the request being undefined since we're not using any express middle-ware to read it.

Updated docs pages

Debugging Webhooks locally - /create-self-service-experiences/setup-backend/webhook/local-debugging-webhook/#creating-a-small-example-server-in-nodejs

[aws-amplify-eu-west-1[bot]]

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