proxy: encode json as we parse rows (#11992)

Serialize query row responses directly into JSON. Some of this code
should be using the `json::value_as_object/list` macros, but I've
avoided it for now to minimize the size of the diff.

Cargo.lock

@@ -5329,6 +5329,7 @@ dependencies = [
  "itoa",
  "jose-jwa",
  "jose-jwk",
+ "json",
  "lasso",
  "measured",
  "metrics",