Allow passing `metadata` to `LangChainAgentNode._run_single`

[dagardner-nv]

Description

• Allows passing arbitrary metadata in to the agent.
• Update a few imports to lower the number of deprecation warnings

Closes #1706

By Submitting this PR I confirm:

• I am familiar with the Contributing Guidelines.
• When the PR is ready for review, new or existing tests cover these changes.
• When the PR is ready for review, the documentation is up to date with these changes.

[mdemoret-nv]

/merge