Implements a JanusMetadataProvider using custom management

[arcade-player]

Some user are reporting that they are unable to fetch metadata from Janus due to fact that the metadata provider is traversing the whole graph.

One solution could be to write a custom JanusMetadataProvider that uses JanusGraphManagement instead of traversing the graph itself

https://stackoverflow.com/questions/49400799/can-we-find-all-the-labels-available-in-a-graph-in-janusgraph/49404838#49404838