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_test.go

@@ -0,0 +1,46 @@
+package backendplugin
+
+import (
+	"sort"
+	"testing"
+
+	"github.com/grafana/grafana/pkg/setting"
+	"github.com/stretchr/testify/require"
+)
+
+func TestPluginSettings(t *testing.T) {
+	t.Run("Should only extract from sections beginning with 'plugin.' in config", func(t *testing.T) {
+		cfg := &setting.Cfg{
+			PluginSettings: setting.PluginSettings{
+				"plugin": map[string]string{
+					"key1": "value1",
+					"key2": "value2",
+				},
+			},
+		}
+
+		ps := extractPluginSettings(cfg)
+		require.Len(t, ps, 1)
+		require.Len(t, ps["plugin"], 2)
+
+		t.Run("Should skip path setting", func(t *testing.T) {
+			cfg.PluginSettings["plugin"]["path"] = "value"
+			ps := extractPluginSettings(cfg)
+			require.Len(t, ps["plugin"], 2)
+		})
+
+		t.Run("Should skip id setting", func(t *testing.T) {
+			cfg.PluginSettings["plugin"]["id"] = "value"
+			ps := extractPluginSettings(cfg)
+			require.Len(t, ps["plugin"], 2)
+		})
+
+		t.Run("Should return expected environment variables from plugin settings ", func(t *testing.T) {
+			ps := extractPluginSettings(cfg)
+			env := ps["plugin"].ToEnv("GF_PLUGIN", []string{"GF_VERSION=6.7.0"})
+			sort.Strings(env)
+			require.Len(t, env, 3)
+			require.EqualValues(t, []string{"GF_PLUGIN_KEY1=value1", "GF_PLUGIN_KEY2=value2", "GF_VERSION=6.7.0"}, env)
+		})
+	})
+}