pgsql: Improve JsonLexContext's freeability

Improve JsonLexContext's freeability

Previously, the JSON code didn't have to worry too much about freeing
JsonLexContext, because it was never too long-lived. With new features
being added for SQL/JSON this is no longer the case. Add a routine
that knows how to free this struct and apply that to a few places, to
prevent this from becoming problematic.

At the same time, we change the API of makeJsonLexContextCstringLen to
make it receive a pointer to JsonLexContext for callers that want it to
be stack-allocated; it can also be passed as NULL to get the original
behavior of a palloc'ed one.

This also causes an ABI break due to the addition of flags to
JsonLexContext, so we can't easily backpatch it. AFAICS that's not much
of a problem; apparently some leaks might exist in JSON usage of
text-search, for example via json_to_tsvector, but I haven't seen any
complaints about that.

Per Coverity complaint about datum_to_jsonb_internal().

Discussion: https://postgr.es/m/20230808174110.oq3iymllsv6amkih@alvherre.pgsql

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1c99cde2f3440c59f582d45b251412c9a9b54f62

Modified Files
--------------
src/backend/utils/adt/json.c | 39 ++++++-----
src/backend/utils/adt/jsonb.c | 13 ++--
src/backend/utils/adt/jsonfuncs.c | 107 ++++++++++++++++---------------
src/bin/pg_verifybackup/parse_manifest.c | 4 +-
src/common/jsonapi.c | 46 +++++++++++--
src/include/common/jsonapi.h | 26 ++++++--
src/include/utils/jsonfuncs.h | 4 +-
7 files changed, 153 insertions(+), 86 deletions(-)