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_test.go

@@ -0,0 +1,43 @@
+package setting
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestPluginSettings(t *testing.T) {
+	cfg := NewCfg()
+	sec, err := cfg.Raw.NewSection("plugin")
+	require.NoError(t, err)
+	_, err = sec.NewKey("key", "value")
+	require.NoError(t, err)
+
+	sec, err = cfg.Raw.NewSection("plugin.plugin")
+	require.NoError(t, err)
+	_, err = sec.NewKey("key1", "value1")
+	require.NoError(t, err)
+	_, err = sec.NewKey("key2", "value2")
+	require.NoError(t, err)
+
+	sec, err = cfg.Raw.NewSection("plugin.plugin2")
+	require.NoError(t, err)
+	_, err = sec.NewKey("key3", "value3")
+	require.NoError(t, err)
+	_, err = sec.NewKey("key4", "value4")
+	require.NoError(t, err)
+
+	sec, err = cfg.Raw.NewSection("other")
+	require.NoError(t, err)
+	_, err = sec.NewKey("keySomething", "whatever")
+	require.NoError(t, err)
+
+	ps := extractPluginSettings(cfg.Raw.Sections())
+	require.Len(t, ps, 2)
+	require.Len(t, ps["plugin"], 2)
+	require.Equal(t, ps["plugin"]["key1"], "value1")
+	require.Equal(t, ps["plugin"]["key2"], "value2")
+	require.Len(t, ps["plugin2"], 2)
+	require.Equal(t, ps["plugin2"]["key3"], "value3")
+	require.Equal(t, ps["plugin2"]["key4"], "value4")
+}