Benchmark alternate Go JSON ser/de packages

[taylor-sutton]

We use the Go standard library one, which is good enough for some purposes, but it's not the most heavily optimized one. For example, https://github.com/goccy/go-json looks interesting, and it also lists a few other options in its README.

One thing I like about this as an area to investigate is that the JSON handling is currently completely opaque to those using wag server, so as long as we keep to the JSON spec, there's very low risk of any observable behavior changing .