Implement the pipeline builder for professor endpoints #279
Description
Professors aggregate endpoints (/professor/courses & /professor/sections) use 2 Mongo pipelines, professorCoursePipeline & professorSectionPipeline for aggregating data.
They are functional, but are hardcoded in each endpoint. Since the 2 pipelines share lots of stage, it would be more managable for us to write a single function that returns the appropriate pipeline based on the types of endpoints.