Alerting: Add function to get all alert rules

[maikelvl]

Credits to @flkhndlr (#144)

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This add a feature to fetch all Alert rules.

It uses the standard API: https://grafana.com/docs/grafana/latest/developers/http_api/alerting_provisioning/#span-idroute-get-alert-rulesspan-get-all-the-alert-rules-_routegetalertrules

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