Add webhook output plugin

[eminano]

This PR adds a webhook output plugin, composed of:

• Subscription store: where the webhook subscriptions are stored. For now this only includes a postgres implementation, but caching will be added in a separate PR to minimise calls to the persistent store and improve the latency of the processor.
• Subscription server: simple HTTP server where users can send requests to register/unregister subscriptions to the wal events. It updates the subscription store.
• Webhook notifier: processes wal events, implementing the wal processor interface. It uses the subscription store to retrieve the relevant subscriptions for the event on input, and triggers a send to the configured server url with the wal data as payload.

A dummy webhook server tool is added. It's intended for validation purposes.

Commits have been split for ease of reviewing.

[exekias]

nice! looking forward to a demo!