Allow forwarding of Auth headers along to datasource

[jknudson]

The altinity clickhouse plugin for Grafana allows forwarding headers related to authentication along to the datasource.

We would like to see this implemented in this plugin as well. Currently, it appears that only static secure headers can be sent in requests to the clickhouse cluster. We'd like to forward along headers related to authentication performed by our auth proxy all the way to the datasource so we can restrict access at the Clickhouse cluster level.

[jknudson]

this would essentially be support for grafana/grafana#87364 for this plugin