Add router chain

**Is your feature request related to a problem? Please describe.**
[Router chain](https://v03.api.js.langchain.com/classes/langchain.chains.RouterChain.html) is necessary to move toward conditional edges.

**Describe the solution you'd like**
Simple router chain implementation, example, and doc.

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context**
I can submit a PR.