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/testdata/test-configs/dashboards-from-disk/dev-dashboards.yaml

@@ -1,7 +1,7 @@
 apiVersion: 1
 
 providers:
-- name: 'general dashboards'
+- name: '$TEST_VAR dashboards'
   orgId: 2
   folder: 'developers'
   folderUid: 'xyz'