chg: dev: Stop prettifying JSON statistics

Passing the JSON_C_TO_STRING_PRETTY flag to
json_object_to_json_string_ext() makes the latter produce prettified
JSON output.  This results in a huge amount of redundant whitespace
being inserted into each HTTP response (whitespace amounts to about 40%
of the entire JSON payload).

The bandwidth cost can be amortized by enabling HTTP compression on the
client side ("Accept-Encoding: deflate"), but that does not affect the
size of data at rest.

Use the JSON_C_TO_STRING_PLAIN flag instead of JSON_C_TO_STRING_PRETTY
to minimize the size of JSON responses sent via the statistics channel.
External tools should be used for prettifying JSON data.

Closes #3304

Merge branch '3304-stop-prettifying-json-statistics' into 'main'

See merge request isc-projects/bind9!10786