Add feature toggle enable field

[oshirohugo]

What this PR does / why we need it:

This PR adds FeatureToggleEnable field to QueryDataRequest to we can forward feature flags from grafana-core to backend plugins.

Special notes for your reviewer:
This is related to this PR:
grafana/grafana#72995

[CLAassistant]

All committers have signed the CLA.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[oshirohugo]

This PR was closed in favor of:
#722