pgsql: Ensure we allocate NAMEDATALEN bytes for names in Index Only Sca

Ensure we allocate NAMEDATALEN bytes for names in Index Only Scans

As an optimization, we store "name" columns as cstrings in btree
indexes.

Here we modify it so that Index Only Scans convert these cstrings back
to names with NAMEDATALEN bytes rather than storing the cstring in the
tuple slot, as was happening previously.

Bug: #17855
Reported-by: Alexander Lakhin
Reviewed-by: Alexander Lakhin, Tom Lane
Discussion: https://postgr.es/m/17855-5f523e0f9769a566@postgresql.org
Backpatch-through: 12, all supported versions

Branch
------
REL_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/e3f9dcabd6f2c4c1845f16ec24b59e0751b055af

Modified Files
--------------
src/backend/executor/nodeIndexonlyscan.c | 95 +++++++++++++++++++++++++--
src/include/catalog/pg_opclass.dat | 7 +-
src/include/nodes/execnodes.h | 4 ++
src/test/regress/expected/index_including.out | 25 +++++++
src/test/regress/sql/index_including.sql | 19 ++++++
5 files changed, 141 insertions(+), 9 deletions(-)