Hotfix: Issues with custom pipelines

[kels271828]

WHAT
Fixed issues related to custom Pipeline and Config classes when reloading from JSON files.

WHY

• Custom pipeline objects were being reloaded as the base Pipeline class; would be missing any custom fields or methods and config would not be validated
• Custom pipeline config objects were being reloaded as base Config class in the stage config; pipeline config fields would not be validated
  • Noticed by Peng when running a stage on Jobmon. In particular, a pipeline config item was not converted from str to Path when loaded from JSON file.

HOW

• pipeline.py:
  • Added type and module attributes to Pipeline
  • Added __init__() function to Pipeline
• main.py:
  • Generalized _get_stage() and _get_custom_stage() methods to work with stages and pipelines; renamed to _get_class() and _get_custom_class()
  • Updated load_pipeline to use _get_class() function rather than returning a base Pipeline object
• config.py: Infer type of _pipeline_config rather than using a base Config object
• advanced_usage.rst: Updated documentation related to custom Config and StageConfig classes

[kels271828]

I wasn't sure where this fix would go (i.e., 1.0.4 vs. 1.1.1), so I haven't updated the version/changelog/etc. Also still working on tests.

[kels271828]

Pulling into release/1.1 instead of as a hotfix to main