Provisioning: Interpolate env vars in provisioning files (#16499)

* Add value types with custom unmarshalling logic

* Add env support for notifications config

* Use env vars in json data tests for values

* Add some more complexities to value tests

* Update comment with example usage

* Set env directly in the tests, removing patching

* Update documentation

* Add env var to the file reader tests

* Add raw value

* Post merge fixes

* Add comment

pkg/services/provisioning/dashboards/config_reader_test.go

@@ -1,6 +1,7 @@
 package dashboards
 
 import (
+	"os"
 	"testing"
 
 	"github.com/grafana/grafana/pkg/log"
@@ -18,8 +19,10 @@ func TestDashboardsAsConfig(t *testing.T) {
 		logger := log.New("test-logger")
 
 		Convey("Can read config file version 1 format", func() {
+			_ = os.Setenv("TEST_VAR", "general")
 			cfgProvider := configReader{path: simpleDashboardConfig, log: logger}
 			cfg, err := cfgProvider.readConfig()
+			_ = os.Unsetenv("TEST_VAR")
 			So(err, ShouldBeNil)
 
 			validateDashboardAsConfig(t, cfg)