Backend Plugins: Plugin configuration using Grafana config (#23451)

Enables adding a section `plugin.<plugin id>` and key/value to
Grafana configuration file which will be converted and sent
as environment variables to the backend plugin.
Also sends some additional environment variables, Grafana
version (GF_VERSION), Grafana edition (GF_EDITION) and 
enterprise license path (GF_ENTERPRISE_LICENSE_PATH).

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

Fixes #21515,

pkg/setting/setting.go

@@ -259,6 +259,7 @@ type Cfg struct {
 	MetricsEndpointDisableTotalStats bool
 	PluginsEnableAlpha               bool
 	PluginsAppsSkipVerifyTLS         bool
+	PluginSettings                   PluginSettings
 	DisableSanitizeHtml              bool
 	EnterpriseLicensePath            string