pgsql: Disallow CREATE STATISTICS on system catalogs

Disallow CREATE STATISTICS on system catalogs

Add a check that CREATE STATISTICS does not add extended statistics on
system catalogs, similarly to indexes etc.  It can be overriden using
the allow_system_table_mods GUC.

This bug exists since 7b504eb282c, adding the extended statistics, so
backpatch all the way back to PostgreSQL 10.

Author: Tomas Vondra
Reported-by: Dean Rasheed
Backpatch-through: 10
Discussion: https://postgr.es/m/CAEZATCXAPrrOKwEsyZKQ4uzzJQWBCt6QAvOcgqRGdWwT1zb%2BrQ%40mail.gmail.com

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/943a113bcb6fb913814ae32cbd4dd1ed29498750

Modified Files
--------------
src/backend/commands/statscmds.c        |  7 +++++++
src/test/regress/expected/stats_ext.out | 12 +++++++-----
src/test/regress/sql/stats_ext.sql      | 12 +++++++-----
3 files changed, 21 insertions(+), 10 deletions(-)