Refactor: context module into a package

[angela-tran]

Closes #2682

This PR addresses:

Naming of the module...

• by introducing a context folder that has a package initialization file (__init__.py) and moving the agency-specific context code into an agency module within the package

... and [naming of the] dict variables

• by removing _context from the module-level variables

Key for the context dict

• by using .value when constructing the dictionary

[github-actions]

Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
benefits/core/context
__init__.py
agency.py
benefits/core/models
transit.py
Project Total

This report was generated by python-coverage-comment-action

[thekaveman]

This looks great 👍

[thekaveman]

I think this can close #2682 since we decided to ignore frozen data classes for now.