feat: add native incident.io Alertmanager receiver

[LukoJy3D]

What this PR does

Mimir already parsed incidentio_configs but never built a notifier, so those receivers silently dropped notifications.

Upstream Alertmanager added a native incident.io receiver in 0.29.0. Mimir started embedding a new enough library in 3.1.0 (Alertmanager v0.31.1; 3.0.x was still v0.28.1). From 3.1.0 onward Mimir unmarshals incidentio_configs, so mimirtool alertmanager get and the v2 alerts API show the receiver as selected.

buildReceiverIntegrations never constructed a notifier from IncidentioConfigs, so those receivers have been a silent no-op since 3.1.0: no integration="incidentio" notification metrics or logs, while Slack/webhook_configs on the same alert still fire.

This wires the existing notify/incidentio notifier (same pattern as MS Teams V2), adds incidentio to allowedIntegrationNames, and rejects url_file / alert_source_token_file so tenant configs stay consistent with Mimir’s *_file secret policy. Inline url + alert_source_token works.

receivers:
  - name: incidentio-notifications
    incidentio_configs:
      - url: https://api.incident.io/v2/alert_events/alertmanager/<source-id>
        alert_source_token: <token>

Until this ships, the working workaround is webhook_configs with Bearer auth to the same URL, yet with high volume of alerts it can easily get rate limited from incident.io side

Checklist

• Tests updated.
• Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]. If changelog entry is not needed, please add the changelog-not-needed label to the PR.
• about-versioning.md updated with experimental features.

[cla-assistant]

All committers have signed the CLA.