Automatically forward core plugin request HTTP headers in outgoing HTTP requests.
Core datasource plugin authors don't have to specifically handle forwarding of HTTP
headers, e.g. do not have to "hardcode" the header-names in the datasource plugin,
if not having custom needs.
Fixes#57065
Grafana would forward the X-Grafana-User header to backend plugin request when
dataproxy.send_user_header is enabled. In addition, X-Grafana-User will be automatically
forwarded in outgoing HTTP requests for core/builtin HTTP datasources.
Use grafana-plugin-sdk-go v0.147.0.
Fixes#47734
Co-authored-by: Will Browne <wbrowne@users.noreply.github.com>
