VMAlertmanagerConfig: add support for Mattermost

[UndeadDemidov]

Is your feature request related to a problem? Please describe

No response

Describe the solution you'd like

Add support for Mattermost config according official Alertmanager spec:
https://prometheus.io/docs/alerting/latest/configuration/#mattermost_config

Describe alternatives you've considered

No response

Additional information

No response

[megative]

I'm currently testing some integration but I believe it should be reviewed and tested precisely before merging.

[megative]

sorry haven't noticed it's already aligned with pr, seems my changes are not relevant anymore