[CT-3557] Define SemanticModel contract in dbt/artifacts

[MichelleArk]

Housekeeping

• I am a maintainer of dbt-core

Short description

As part of an effort to move any ArtifactMixin subclasses upstream of other core modules into the top -level dbt/artifacts directory, we need to move SemanticModel, which WritableManifest depends on.

Acceptance criteria

The SemanticModel classes should be split into data + functional components, where data contracts are defined upstream in dbt/artifacts.

Impact to Other Teams

N/A - should be a refactor.

Will backports be required?

nope

Context

No response