Major interface and Metabase client refactor

[gouline]

• Remove DbtInterface and MetabaseInterface (call DbtReader and MetabaseClient directly instead)
• Manifest parser is now the only dbt parser available: this was a long time coming, project/folder parser was no longer maintained for many versions, so it's being removed before the major version
• Move CLI code into main.py to simplify running locally (e.g. via python3 -m dbtmetabase)
• Minimise duplication in CLI setup by initialising DbtReader and MetabaseClient once
• Simplify Metabase parameters (e.g. host/use_http becomes url)
• Refactor MetabaseClient into job-specific context, to improve separation of concern
• Minor consistency and typing fixes/improvements

[gouline]

Fixes #185