[CT-3548] Move WritableManifest to 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 WritableManifest. This will pull in many other dependencies - notably contracts.graph.nodes.

This ticket is just to get the initial WritableArtifact in dbt/artifacts without moving all the upstream dependencies to get the foundation set.

Acceptance criteria

A spike report that highlights any potential approaches and their tradeoffs, as well as a summary of any potential blockers encountered.

Impact to Other Teams

N/A - should be a refactor.