feat: Grafana Oncall Plugin

[Tonkonozhenko]

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[Tonkonozhenko]

@klesh @abeizn @mintsweet may I ask you to review the pull request? for me, it has already been working for a month

[klesh]

Hi, @Tonkonozhenko Thanks for your contribution.
Would you mind updating the PR description to describe your work and help others understand the context of the plugin? Thanks.

Please assume that others may not know the Grafana Oncall at all, so, a brief introduction and links to details would help, and it would be great to know what entities or data are currently supported and what is the next step if any.