Summary:
Escape non-printable characters in JsonWriter when writing to the UI, or files.

Fixes #26413
Jira: DB-15775

Test Plan:
Manually tested with http://127.0.0.10:7000/api/v1/xcluster

Without fix it was emitting:
`"start_key":["牧"],"end_key":[""]`

With the fix it now emits:
`"start_key":["\\xaa\\xaa"],"end_key":[""]`

Reviewers: jhe

Reviewed By: jhe

Subscribers: yql, ybase

Differential Revision: https://phorge.dev.yugabyte.com/D42559