Port dbt-core base adapter into dbt-adapter

[colin-rogers-dbt]

We will need to port dbt/adapters into dbt-adapter.

implementation notes:

• we want to preserve existing import path (i.e. dbt.adapters.relation)
• we need to port functional/unit tests as part of this and assess whether we need additional tests (create tickets not implement) or if we can delete them

acceptance criteria:

• functional base adapter
• replicates existing testing
• tickets for adding/removing tests