Create protocol abstraction for SemanticModel

[QMalcolm]

We want a standard definition of what a valid SemanticModel implementation should have. This allows for a shared definition to be understood in MetricFlow and dbt-core of what anything implementing the SemanticModel protocol will make available without MetricFlow and dbt-core needing to import each other. This should use the new Protocol type which exists in python 3.8+

[WilliamDee]

from __future__ import annotations

from typing import Optional, Protocol, Sequence

from dbt_semantic_interfaces.protocols.dimension import Dimension
from dbt_semantic_interfaces.protocols.entity import Entity
from dbt_semantic_interfaces.protocols.measure import Measure
from dbt_semantic_interfaces.protocols.metadata import Metadata

class NodeRelation(Protocol):
    """Path object to where the data should be"""

    alias: str
    schema_name: str
    database: Optional[str]
    relation_name: str

class SemanticModel(Protocol):
    """Describes a semantic model"""

    name: str
    description: Optional[str]
    node_relation: NodeRelation

    entities: Sequence[Entity]
    measures: Sequence[Measure]
    dimensions: Sequence[Dimension]

    metadata: Optional[Metadata]