Fix a lot of mypy things, add a number of adapter-ish modules to it
Split relations and columns into separate files
split context.common into base + common
 - base is all that's required for the config renderer
Move Credentials into connection contracts since that's what they really are
Removed model_name/table_name -> consolidated to identifier
 - I hope I did not break seeds, which claimed to care about render(False)
Unify shared 'external' relation type with bigquery's own
hack workarounds for some import cycles with plugin registration and config p
arsing
Assorted backwards compatibility fixes around types, deep_merge vs shallow merge
Remove APIObject

add typing extensions module to setup.py
Update changelog