feat: support optional flag in from_ extractors

[mgdemers]

Support optional from_ extractors when DAG modules' targets do not reference an extractor.

Changes

• Added optional flag to ExtractorFactory
• When no target node is identified during generate_nodes, return an empty list instead of raising ValueError when flag is set.
• Add unit test for an optional extractor not referenced by a DAG module

How I tested this

• Unit tests

Notes

Discussion in #1551

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.

[skrawcz]

I would add a bit more here.

Suggested change

	:param optional: Whether missing targets should raise exceptions or skip being loaded.
	:param optional: Whether missing targets should raise exceptions or skip being loaded. Optional=True will mean that if the target does not exist in your graph, we skip try to inject this into the graph.

[mgdemers]

Added - I also added a note on the materialization docs about specifying optional data loaders.

[skrawcz]

looks good. can you update the materialization docs too? A comment or example snippet should suffice.