Add static shared secret to webhooks

Implement integration with Azure Event Hub.

Basically it's a webhook with details specified in the following documentation:

https://learn.microsoft.com/en-us/rest/api/eventhub/generate-sas-token
https://learn.microsoft.com/en-us/rest/api/eventhub/send-event

Original request:

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Currently the webhooks in the Flagsmith platform allow users to set a secret which adds a header consisting of the payload data hashed using the sha256 algorithm with the given secret.

To be able to serve other platforms (e.g. Azure Event Hub) it would be useful to be able to separately (and optionally) configure a static shared secret that is added as another header on all requests made to the Webhook.