Remove mandatory variable Definitions and allow access token in header

[salujaharkirat]

For queries which don't require variables the code was breaking as _context.variableDefinitions would be null and upon reading variableDefinitions would throw error.

Adding following checks:-

1. Reading variableDefinitions only if _context.variableDefinitions is present.
2. If variables is set then assigning to final query else assigning null

Auth token changes:-

1. allowing option to pass auth token in the api calls.