Read parsed config from json file

[zzeekk]

Parsing big configurations with Javascript takes some time, e.g. >10s for configurations with several hundred objects.
The Visualizer should support reading a parsed and resolved configuration from a json file. This file can be produced by Java Hocon parser during deployment, see also smart-data-lake/smart-data-lake#575.

Default should be to search for exportedConfig.json first in visualizer base url, then searching for Hocon configuration as already implemented.