[CT-3538] [Adapter Decoupling] Add dbt-common as dependency and remove dbt/common subdir

[MichelleArk]

Housekeeping

• I am a maintainer of dbt-core

Short description

As part of the effort to decouple dbt/adapters from core, we have created a new top-level directory that contains utilities used by both dbt-adapter and dbt-core. The code from dbt/common can be removed once https://github.com/dbt-labs/dbt-common is set up to contain it. The code from dbt/common can be removed once https://github.com/dbt-labs/dbt-common is set up to contain it, and can be replaced by a new dependency in setup.py

Acceptance criteria

• Delete dbt/common
• Delete tests/unit/common
• Add dbt-common to setup.py (as a GH dependency until release process for dbt-common is ironed out)
• Remove any dependencies / third party stubs that are no longer necessary once dbt/common is moved out

Suggested Tests

N/A - tests moving to dbt/common as part of the refactor

Impact to Other Teams

Adapter implementations will no longer be able to import dbt.common directly from dbt-core as a result of this change, meaning 1P adapters will need to point to dbt-common as well as part of their migration plan. This is already accounted for in 1P Adapter Migrations in #8917.

Will backports be required?

No

Context

No response