pgsql: Only allow returning string types or bytea from json_serialize

Only allow returning string types or bytea from json_serialize

These are documented to be the allowed types for the RETURNING clause,
but the restriction was not being enforced, which caused a segfault if
another type was specified. Add some testing for this.

Per report from a.kozhemyakin

Backpatch to release 15.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3c633f32b9c712cc0b4c8d946f0eeae04a3ff51a

Modified Files
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src/backend/parser/parse_expr.c       | 17 +++++++++++++++++
src/test/regress/expected/sqljson.out | 10 ++++++++++
src/test/regress/sql/sqljson.sql      |  5 +++++
3 files changed, 32 insertions(+)