Support multi-tenant Mimir Alermanager as external Alertmanager

[pgillich]

Is your feature request related to a problem? Please describe.
I'd like to use multi-tenant Mimir Alermanager as external Alertmanager. It's possible to set Mimir Alertmanager URL at ruler.alertmanager_url and other client configs at ruler.alertmanager_client, but unable to set X-Scope-OrgId HTTP header to send tenant information to Mimir Alertmanager.

Describe the solution you'd like
The solution should be similar to ruler.remote_write.add_org_id_header. If ruler.alertmanager_client.add_org_id_header = true, the tenant ID (userID in source code) will be added to the HTTP header. Default value: false, for backward-compatible config.

Additional changes:

• Automatic tests
• CLI and Helm config parameter
• Loki operator support

Describe alternatives you've considered
A list of HTTP header pairs may be set at ruler.alertmanager_client.headers, but the X-Scope-OrgId header will be same for all Loki tenants, which is not a good behavior.

Additional context
https://grafana.com/docs/loki/latest/configure/#ruler

If the solution is accepted, I can implement it.