BE: Forward proxied events to self-hosted instance

In #321401 (closed) we did investigation and created a prototype for a proxy of the Atlassian connect app that could be used to configure self-managed GitLab instances.

This issues is to implement the forwarding events described here: #321401 (closed)

Specifically:

Once an installation is updated with a instance_url we will send a installed event webhook to the instance to pretend that the app was installed on Jira. Whenever we receive events for installations that have a instance_url we need to forward them to the instance.

There are two types of event hooks:

• installed: Will be sent when the app is installed or updated
• uninstalled: Will be sent when the app is uninstalled

When receiving an installed hook and the JiraConnectInstallation already exists, we can consider it an update of the app. The initial installed hook will be added as part of #325281 (closed)