Webhooks should send a X-Gitlab-Instance header

Problem to solve

As a webhook developper,

I would like a "X-Gitlab-Instance" header added to webhooks events (eg: "X-Gitlab-Instance: gitlab.com", "X-Gitlab-Instance: gitlab.example.com")

In order to process requests differently if the webhook server is configured to handle different instances.

Intended users

Webhook developpers

Links / references

https://docs.gitlab.com/ee/user/project/integrations/webhooks.html#overview