Update SemanticManifestValidator to expect a generic bound by the SemanticManifest Protocol

[QMalcolm]

Currently the SemanticManifestValidator (currently named ModelValidator) expects a UserConfiguredModel which is a concrete object. Initially we thought we should move to it expecting SemanticManifest protocol. However, in dbt-core we'll be writing nodes which extend the protocol definition. If we want to be able to write validation rules that can operate on the extensions and guarantee type safety, then we need to take it a step further. Thus the SemanticManifestValidator should instead operate on a generic bound by the SemanticManifest protocol.

Something like...

from typing import TypeVar
from dbt_semantic_interfaces.protocols import SemanticManifest


T = TypeVar("T", bound="SemanticManifest")
class SemanticManifestValidator:
  ...

  def validate(self, semantic_manifest: T) -> ValidationResults:
    ...