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_plugins.go

@@ -0,0 +1,25 @@
+package setting
+
+import (
+	"strings"
+
+	"gopkg.in/ini.v1"
+)
+
+// PluginSettings maps plugin id to map of key/value settings.
+type PluginSettings map[string]map[string]string
+
+func extractPluginSettings(sections []*ini.Section) PluginSettings {
+	psMap := PluginSettings{}
+	for _, section := range sections {
+		sectionName := section.Name()
+		if !strings.HasPrefix(sectionName, "plugin.") {
+			continue
+		}
+
+		pluginID := strings.Replace(sectionName, "plugin.", "", 1)
+		psMap[pluginID] = section.KeysHash()
+	}
+
+	return psMap
+}