Better Webhook support

[deepforest]

It would be very helpful to make the Integration Webhook more open so for example we can send notifications over Google Chat on flag changes: Add, Remove, update.

Currently, using the exiting Webhook integration we need to create our own REST endpoint, host it somewhere and design it to accept the
very specific JSON schema sent by Flagsmith.

If you look at SignalFx as an example, they provide a very clever Webhook integration, where you can select/project whatever you want from the SignalFx payload, map it to JSON schema forged in-place and send it over HTTP.

The minimum is to extend the integration support with common services, such as sending notifications to Google Chat.

[deepforest]

Added by mistake.