copy: support json format

[dbist]

Is your feature request related to a problem? Please describe.
Testing DuckDB Postgres connector which relies on libpq and which translates queries to COPY (SELECT *) TO STDOUT (FORMAT json);`

COPY (SELECT * FROM test LIMIT 5) TO STDOUT (FORMAT json);    
invalid syntax: statement ignored: at or near "json": syntax error: unimplemented: this syntax
SQLSTATE: 0A000
DETAIL: source SQL:
COPY (SELECT * FROM test LIMIT 5) TO STDOUT (FORMAT json)
                                                    ^
HINT: You have attempted to use a feature that is not yet implemented.
See: https://go.crdb.dev/issue-v/96590/v23.1

Describe the solution you'd like
support copy with json

Describe alternatives you've considered
unfortunately DuckDB does not support csv or text

This is CockroachDB 23.1.1

Related issues: #103542 and #103544

Jira issue: CRDB-28070