[CT-2521] Define SemanticModel node for the Manifest

[QMalcolm]

There are two new node types being added to the manifest as part of the MetricFlow integration work: SemanticModel nodes and MetricNodes. This ticket addresses the SemanticModel nodes. In dbt-semantic-interfaces we're adding a protocol of a SemanticModel, which will have the shared standard definition of a SemanticModel. In dbt-core we need to compose this protocol with base node classes to define dbt-core's representation of a SemanticModel. Additionally there are some additional fields we should expose as defined in #7456

Blocked by:

• [CT-2496] [Spike] Determine best practices for composing nodes with python Protocols #7470
• Create protocol abstraction for SemanticModel dbt-semantic-interfaces#17