Refactoring to make testing easier & adding tests

[syou6162]

Why

• ref: https://github.com/z3z1ma/dbt-osmosis/issues/93

What

@z3z1ma

• To adding tests easier, I split functions to another file(src/dbt_osmosis/core/column_level_knowledge_propagator.py)
  • Defined types for ColumnLevelKnowledge and Knowledge to understand code easier
• Adding some tests
  • In this pull request, we are only adding functions for which tests are relatively easy to add (to avoid making the differences in this pull request too large and difficult to review)
  • We intend to add more detailed tests and tests for other functions in subsequent pull requests
• Run pytest on GitHub Actions