Feature: Slack/Mattermost receiver for Observability alerting

[viktorgt]

Problem description

Prometheus Alertmanager supports Mattermost/Slack directly, it has a dedicated config.

None of that is reflected in the STACKIT Terraform resource. stackit_observability_instance.alert_config
exposes only a subset of Alertmanager's receivers: email, OpsGenie, and a generic
webhook (with google_chat / ms_teams formatting flags). There is no slack_configs,
no Mattermost receiver, and no flag that produces a Mattermost-compatible body. We run
alerting through Mattermost, so in practice the only channel that reaches the team is
email, which is easy to miss and has no per-channel routing.

Proposed solution

Introduces native Mattermost/Slack receiver support in Observability alerting,
exposing Alertmanager's mattermost_configs (and/or slack_configs) through the
Observability API and the stackit_observability_instance.alert_config Terraform
resource, the same way email_configs, opsgenie_configs, and webhooks_configs are
exposed today.