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/testdata/test-configs/correct-properties/correct-properties.yaml

@@ -1,5 +1,5 @@
 notifiers:
-  - name: default-slack-notification
+  - name: $TEST_VAR-slack-notification
     type: slack
     uid: notifier1
     org_id: 2
@@ -39,4 +39,4 @@ delete_notifiers:
     org_id: 0
     uid: "notifier3"
   - name: Deleted notification with whitespaces in name
-    uid: "notifier4"
+    uid: "notifier4"