pgsql: Move privilege check for SET SESSION AUTHORIZATION.

Move privilege check for SET SESSION AUTHORIZATION.

Presently, the privilege check for SET SESSION AUTHORIZATION is
performed in session_authorization's assign_hook. A relevant
comment states, "It's OK because the check does not require catalog
access and can't fail during an end-of-transaction GUC
reversion..." However, we plan to add a catalog lookup to this
privilege check in a follow-up commit.

This commit moves this privilege check to the check_hook for
session_authorization. Like check_role(), we do not throw a hard
error for insufficient privileges when the source is PGC_S_TEST.

Author: Joseph Koshakow
Discussion: https://postgr.es/m/CAAvxfHc-HHzONQ2oXdvhFF9ayRnidPwK%2BfVBhRzaBWYYLVQL-g%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9987a7bf34061ed5cffc4e5113da056358976e94

Modified Files
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src/backend/commands/variable.c | 32 ++++++++++++++++++++++++++++----
src/backend/utils/init/miscinit.c | 30 ++++++++++++------------------
src/include/miscadmin.h | 1 +
3 files changed, 41 insertions(+), 22 deletions(-)