Add AgentRuntime

[sydney-runkle]

Privileged issue

• I am a LangChain maintainer, or was asked directly by a LangChain maintainer to create an issue here.

Issue Content

Enhanced w/ things like agent name, perhaps config, state, etc

Analogous to ToolRuntime, sorta.

To be spec-ed out further before I take this up

[sydney-runkle]

Got a start here, but now out of date: #33689