Add support for lexicon data sources

[jkerr5]

Building on the work done in #111, add a "lexicons" data source that allows the pipeline plan to come from one or more lexicons.

See https://docs.marklogic.com/op.fromLexicons?q=fromLexicons&v=9.0&api=true

The data source specification would look something like

        {
          "source" : "lexicons",
          "fields" : {
            "field1" : { 
              "scalarType" : "string",
              "reference" : "..." 
            },
            "field2" : { 
              "scalarType" : "string",
              "reference" : "..." 
            }
          }
        }

Where reference is a serialized cts reference that can be parsed by https://docs.marklogic.com/cts.referenceParse The scalarType property my be redundant though as the type would be in the serialized reference.

[jkerr5]

This would need to move to https://github.com/marklogic-community/marklogic-geo-data-services if we were to implement this. It is unlikely we will implement it before refactoring GDS completely though so closing this for now.