[CT-2545] Add method to populate relation fields of nodes

[stu-k]

Description

As part of the dbt clone work, we need a method which populates the "other" relation fields introduced in #7550.

Jeremy has a draft PR up that accomplishes this, but we are not beholden to that implementation.

Acceptance criteria

• A function/method has been created that adds node relations to the "other" relation fields created on reffable nodes in [CT-2544] Add "other" relation attribute to reffable node classes #7550
• The added function has tests

Dependencies

• [CT-2544] Add "other" relation attribute to reffable node classes #7550 must be completed before this ticket

Links from the draft PR

Adding a method to Manifest that takes an "other" manifest, creates relations to the "other" manifest, and fills the nodes in the original manifest with these relations

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

Lines 1037 to 1058 in 5f08d70

	def add_from_artifact(
	self,
	other: "WritableManifest",
	) -> None:
	"""Update this manifest by *adding* information about each node's location
	in the other manifest.
	Only non-ephemeral refable nodes are examined.
	"""
	refables = set(NodeType.refable())
	for unique_id, node in other.nodes.items():
	current = self.nodes.get(unique_id)
	if current and (node.resource_type in refables and not node.is_ephemeral):
	other_node = other.nodes[unique_id]
	state_relation = RelationalNode(
	other_node.database, other_node.schema, other_node.alias
	)
	self.nodes[unique_id] = current.replace(state_relation=state_relation)
	# Rebuild the flat_graph, which powers the 'graph' context variable,
	# now that we've deferred some nodes
	self.build_flat_graph()