Webhook Receiver should support custom headers

[njohnstone2]

The webhook receiver should have the ability to add arbitrary headers to a http request.

When configuring a webhook contact point it is common that the webhook receiver requires additional arbitrary headers to process the request (e.g. "X-" headers).

This issue would also require a UI change in grafana to allow the additional headers to be defined in the UI

[armandgrillet]

The challenge is that we want to unify our receivers with the ones in the Prometheus Alertmanager repository in the long term, and this issue + PR go against that. You should propose the idea to https://github.com/prometheus/alertmanager/issues instead.

[armandgrillet]

Related PRs:

• added support for extra headers on the webhook receiver #135
• Alerting: Webhook contact point support custom headers grafana#77668

This does not change the fact that this should first be done in the Prometheus Alertmanager repository so that we do not go against upstream.