grafana-plugin-model is legacy and is replaced by new backend 
plugins SDK and architecture. Renderer is not part of SDK and 
we want to keep it that way for now since it's highly unlikely there 
will be more than one kind of renderer plugin.
So this PR adds support for renderer plugin v2.
Also adds support sending a Device Scale Factor parameter to the 
plugin v2 remote rendering service and by that replaces #22474.
Adds support sending a Headers parameter to the plugin v2 and
remote rendering service which for now only include 
Accect-Language header (the user locale in browser when using 
Grafana), ref grafana/grafana-image-renderer#45.
Fixes health check json details response.
Adds image renderer plugin configuration settings in defaults.ini 
and sample.ini.

Co-Authored-By: Arve Knudsen <arve.knudsen@gmail.com>