Support passing the complete JSON document as rstring attribute

[brandtol]

Currently the JSON document to insert in ES is created from the attribute names and values in the input tuples. Only primitive streams attribute types are allowed. If the upstream operators use other toolkits/functions to create the JSON document directly as a single string, it would be useful if the ElasticsearchIndex operator could handle that string as input.

Tasks to do:

• add paramter 'documentAttribute' that specifies the input attribute holding the JSON document
• investigate how to handle the timestamp related attributes in that case
• add testcase