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.