Derby Properties should report which data source provides attributes

[egonw]

Currently, users of the webservice have to guess which data source has attributes, see these two issues:

• https://github.com/bridgedb/bridgedb-webservice/issues/1
• https://github.com/bridgedb/BridgeDbWebservice/issues/3

It would be nice of the Properties returned by a Derby file actually says which "root" identifiers actually have attributes. For example, for the metabolite ID mapping database, attributes are distributed over HMDB and ChEBI, see the second issue above. cc @DeniseSl22