[SPIKE] [CT-2651] Add SemanticModels and Metrics to the DAG

[QMalcolm]

We've added SemanticModel and Metric nodes to the manifest, but we haven't added them to the dag. We need to get them added to the dag so that visualizations based on the DAG include SemanticModels and Metrics.

Note that inclusion of the SemanticModel node in the dag should include tested support for partial parsing. After initial implementation, changes to the semantic_model section of schema files is not triggering partial (or any) re-parse, as it should.

[jtcohen6]

@QMalcolm Are you thinking about the visualization in dbt-docs? There & elsewhere, the main requirement from within dbt-core is that these resource types are included in manifest.json and the output of dbt list, with requisite information in their depends_on.nodes.

Edit: We also want to make sure that metrics and semantic_models appear in the flat_graph → graph context variable

[jtcohen6]

Should metrics and semantic_models land in the same top-level key (semantic_nodes) of the manifest + graph context var? I think maybe!