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/plugins/backendplugin/plugin_settings.go

@@ -0,0 +1,39 @@
+package backendplugin
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/grafana/grafana/pkg/setting"
+)
+
+type pluginSettings map[string]string
+
+func (ps pluginSettings) ToEnv(prefix string, hostEnv []string) []string {
+	env := []string{}
+	for k, v := range ps {
+		env = append(env, fmt.Sprintf("%s_%s=%s", prefix, strings.ToUpper(k), v))
+	}
+
+	env = append(env, hostEnv...)
+
+	return env
+}
+
+func extractPluginSettings(cfg *setting.Cfg) map[string]pluginSettings {
+	psMap := map[string]pluginSettings{}
+	for pluginID, settings := range cfg.PluginSettings {
+		ps := pluginSettings{}
+		for k, v := range settings {
+			if k == "path" || strings.ToLower(k) == "id" {
+				continue
+			}
+
+			ps[k] = v
+		}
+
+		psMap[pluginID] = ps
+	}
+
+	return psMap
+}