Warmer Function

Create infrastructure for open-next's warmer function. Follow guidance [here](https://github.com/sst/open-next/blob/main/README.md#warmer-function) and reference implementation [here](https://github.com/sst/sst/blob/master/packages/sst/src/constructs/NextjsSite.ts#L250-L293).