Support complex filters in variables

[dmichel1]

Currently the ad-hoc filter variable doesn't support heroic's complex filter syntax.

Users of this plugin would like to set filter for things like role = foo or role = bar.

We need to investigate more into how the ad-hoc filter variable works and potentially work with Grafana in extending this (or maybe its poorly documented and actually implemented). Or use a different type of variable (custom variable?) and use that in our plugin.