feat: add keep_firing_for support to alerting rules

[fgouteroux]

What this PR does / why we need it:

Adding the keep_firing_for in alerting rule.

Which issue(s) this PR fixes:
Fixes #7450

Special notes for your reviewer:

I copied everything from grafana/mimir#4099 thanks to @pracucci.

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• For Helm chart changes bump the Helm chart version in production/helm/loki/Chart.yaml and update production/helm/loki/CHANGELOG.md and production/helm/loki/README.md. Example PR
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

[fgouteroux]

@cyriltovena @dannykopping what do you think about this ?

[vladst3f]

we'd also find this useful!

[fgouteroux]

Any feedback grafana team ?

[fgouteroux]

Still no response about this PR ?

[maffelbaffel]

This would be such a valuable addition to Loki!

[fgouteroux]

Any feedback grafana team ?

[fgouteroux]

may @dannykopping could help for the review ?