Support named schema

[jcbiddle]

Named schema can currently be created via the Database.set_options method, but there is no way to execute a select query with a non-default schema. As far as I can tell, this is supported by the HTTP API (https://stardog-union.github.io/http-docs/#tag/Reasoning/operation/queryPostWithReasoning), so should hopefully be an easy change to the Connection.select method.