This is the first PR for pipelines support, adding the base structure for the new feature:

• added PipelineSource class to create a new pipeline pointed at a resource
  • added client.pipeline() to create a new PipelineSource object
• added pipeline_expressions file to hold set of expressions
  • Currently just holding base Expr and Constant
• added pipeline_stages file to hold set of stages.
  • Currently just holding base Stage, Collection, and GenericStage
• added BasePipeline, Pipeline, and AsyncPipeline to chain together sequences of stages
  • implemented pipleine.execute() in sync and async classes
• added PipelineResult to expose resulting data back to the user

This PR contains unit tests. System tests will come later, when we have more stages