environmentd: convert json numbers to strings

[maddyblue]

This used to happen but regressed at some point.

Fixes MaterializeInc/database-issues#6508

Motivation

• This PR fixes a recognized bug.

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered.
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• This PR includes the following user-facing behavior changes:
  • Change the HTTP and WebSocket JSON interfaces of the sql API to format numeric results as strings instead of JSON numbers.

[ParkMyCar]

This approach works for me, just two thoughts:

1. What about websockets, it looks like they directly create a SqlResult::Rows without calling this method.
2. What do you think about making this change as part of the ToJson impl for TypedDatum?

[maddyblue]

Ah! SUBSCRIBE over ws didn't use this. Ergh. Yes, putting this into TypedDatum.json is a way better choice. Added code for that.

[antiguru]

This breaks the memory visualization because it now compares integers with strings.