Add return_keys parameter for Send to filter returned state

[nick-youngblut]

Checked

• I searched existing ideas and did not find a similar one
• I added a very descriptive title
• I've clearly described the feature request and motivation for it

Feature request

It would be helpful to filter the returned state from Send to certain state variables (e.g., just messages) versus having to return the entire state for each sub-workflow called by Send.

Motivation

While there is probably an existing way to filter the state returned from each Send call, the docs do not describe this.
Including such a filtering parameter directly in Send would make it easy for anyone to quickly filter the returned state from each sub-workflow.

Proposal (If applicable)

No response

[nick-youngblut]

I've moved this to the LangGraph discussion board: #30386