adds a langserve/langgraph example with code execution

[charlesfrye]

This example shows off the use of Sandboxes for securely executing code from LLMs and how to serve a LangServe application with asgi_app.

It comprises a LangGraph-defined agent that has access to the LangChain Expression Language docs and answers questions about LCEL by providing code samples.

Type of Change

• New example

Checklist

• Example is testable in synthetic monitoring system, or lambda-test: false is added to example frontmatter (---)
  • Example is tested by executing with modal run or an alternative cmd is provided in the example frontmatter (e.g. cmd: ["modal", "deploy"])
  • Example is tested by running with no arguments or the args are provided in the example frontmatter (e.g. args: ["--prompt", "Formula for room temperature superconductor:"]
• Example is documented with comments throughout, in a Literate Programming style.
• Example does not require third-party dependencies to be installed locally
• Example pins its dependencies
  • Example pins container images to a stable tag, not a dynamic tag like latest
  • Example specifies a python_version for the base image, if it is used
  • Example pins all dependencies to at least minor version, ~=x.y.z or ==x.y
  • Example dependencies with version < 1 are pinned to patch version, ==0.y.z

Outside contributors

You're great! Thanks for your contribution.

[mwaskom]

Nit but leaving a TODO(charles) would have been a good idea :)