[CT-2544] Add "other" relation attribute to reffable node classes

[stu-k]

Description

As part of the work for clone #7256, reffable nodes need to hold a reference to their "other" counterparts in other schemas.

Jeremy already has a draft PR up that accomplishes this and uses the name "state_relation" as seen in the links below, however we are not beholden to this name.

As always, the language team has the final say in all things artifacts so make sure to get their sign off on whatever approach is taken.

Dependencies

Adapter work from #7549 will be dependent on the work in this ticket.

Acceptance Criteria

• The reffable node classes (i.e. ModelNode, SeedNode, and SnapshotNode) have a field that stores a relation to an "other" counterpart in another schema
• This new attribute is not written to manifest.json
• This new attribute is somehow tested

Links from the draft PR

Separating RelationalNode from ParsedNodeMandatory:

dbt-core/core/dbt/contracts/graph/nodes.py

Lines 231 to 243 in 5f08d70

	@dataclass
	class RelationalNode(HasRelationMetadata):
	alias: str
	@property
	def identifier(self):
	return self.alias
	@dataclass
	class ParsedNodeMandatory(GraphNode, RelationalNode, Replaceable):
	checksum: FileHash
	config: NodeConfig = field(default_factory=NodeConfig)

Adding a relation to a reffable node:

dbt-core/core/dbt/contracts/graph/nodes.py

Lines 570 to 574 in 5f08d70

	@dataclass
	class ModelNode(CompiledNode):
	resource_type: NodeType = field(metadata={"restrict": [NodeType.Model]})
	access: AccessType = AccessType.Protected
	state_relation: Optional[RelationalNode] = None

Serializing the new relation attribute out of manifest writing:

dbt-core/core/dbt/contracts/graph/manifest.py

Lines 1256 to 1262 in 5f08d70

	def __post_serialize__(self, dct):
	for unique_id, node in dct["nodes"].items():
	if "config_call_dict" in node:
	del node["config_call_dict"]
	if "state_relation" in node:
	del node["state_relation"]
	return dct

Adding the new relation attribute to required node keys for manifest tests:

dbt-core/test/unit/test_manifest.py

Lines 91 to 92 in 5f08d70

	"state_relation",
	}