[Artifacts] Define UnitTestDefinition resource in dbt/artifacts/resources

[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 UnitTestDefinition, which WritableManifest depends on.

Acceptance criteria

UnitTestDefinition (and its necessary dependencies) are moved to a new dbt/artifacts directory. dbt/artifacts should have no additional dependencies on dbt.core except for from contracts.nodes which are actively being refactored.

UnitTestDefinition should be split into data & functional interfaces, defined in dbt/artifacts in dbt/core respectively. Additionally, any inheritance hierarchy between concrete node classes should be flattened as part of this spike.

Should follow initial pattern set in #9379

Suggested Tests

N/A - refactor of existing resource definitions, existing serialization tests should cover us (but this should be confirmed)

Impact to Other Teams

N/A

Will backports be required?

no

Context

No response