[sensors-2] sensor def
Sets up a sensor definition, inheriting from the abstract job type.
Currently all jobs are defined by:
At evaluation time, sensors return a should_execute boolean, a run_config dict, and a tags dict.
An alternative implementation has a sensor defined by a single job params function, which returns a list of JobParams (containing a run config dict and a tags dict). Will send out in a separate diff for comparison.
Test Plan: bk
Reviewers: schrockn, dgibson, sandyryza
Reviewed By: dgibson
Differential Revision: https://dagster.phacility.com/D5019