Add x grafana orgid header

[MaoMaoCake]

Adds support for X-Grafana-OrgId header. Addresses #1726

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[weisdd]

@MaoMaoCake thanks for the PR, but, as it was explained in the issue that you referenced (#1726 (comment)) and in the official documentation, we do not intend to support organizations.
The only option that we're open to is adding generic support for custom HTTP headers.