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/notifiers/config_reader.go

@@ -63,7 +63,7 @@ func (cr *configReader) parseNotificationConfig(path string, file os.FileInfo) (
 		return nil, err
 	}
 
-	var cfg *notificationsAsConfig
+	var cfg *notificationsAsConfigV0
 	err = yaml.Unmarshal(yamlFile, &cfg)
 	if err != nil {
 		return nil, err