Introduce SYSTEM_USER

SYSTEM_USER is a reserved keyword of the SQL specification that,
roughly described, is aimed at reporting some information about the
system user who has connected to the database server.  It may include
implementation-specific information about the means by the user
connected, like an authentication method.

This commit implements SYSTEM_USER as of auth_method:identity, where
"auth_method" is a keyword about the authentication method used to log
into the server (like peer, md5, scram-sha-256, gss, etc.) and
"identity" is the authentication identity as introduced by 9afffcb (peer
sets authn to the OS user name, gss to the user principal, etc.).  This
format has been suggested by Tom Lane.

Note that thanks to d951052, SYSTEM_USER is available to parallel
workers.

Bump catalog version.

Author: Bertrand Drouvot
Reviewed-by: Jacob Champion, Joe Conway, Álvaro Herrera, Michael Paquier
Discussion: https://postgr.es/m/7e692b8c-0b11-45db-1cad-3afc5b57409f@amazon.com

src/backend/parser/gram.y

@@ -743,7 +743,7 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 	SERIALIZABLE SERVER SESSION SESSION_USER SET SETS SETOF SHARE SHOW
 	SIMILAR SIMPLE SKIP SMALLINT SNAPSHOT SOME SQL_P STABLE STANDALONE_P
 	START STATEMENT STATISTICS STDIN STDOUT STORAGE STORED STRICT_P STRIP_P
-	SUBSCRIPTION SUBSTRING SUPPORT SYMMETRIC SYSID SYSTEM_P
+	SUBSCRIPTION SUBSTRING SUPPORT SYMMETRIC SYSID SYSTEM_P SYSTEM_USER
 
 	TABLE TABLES TABLESAMPLE TABLESPACE TEMP TEMPLATE TEMPORARY TEXT_P THEN
 	TIES TIME TIMESTAMP TO TRAILING TRANSACTION TRANSFORM
@@ -15239,6 +15239,13 @@ func_expr_common_subexpr:
 				{
 					$$ = makeSQLValueFunction(SVFOP_SESSION_USER, -1, @1);
 				}
+			| SYSTEM_USER
+				{
+					$$ = (Node *) makeFuncCall(SystemFuncName("system_user"),
+											   NIL,
+											   COERCE_SQL_SYNTAX,
+											   @1);
+				}
 			| USER
 				{
 					$$ = makeSQLValueFunction(SVFOP_USER, -1, @1);
@@ -17120,6 +17127,7 @@ reserved_keyword:
 			| SESSION_USER
 			| SOME
 			| SYMMETRIC
+			| SYSTEM_USER
 			| TABLE
 			| THEN
 			| TO
@@ -17500,6 +17508,7 @@ bare_label_keyword:
 			| SYMMETRIC
 			| SYSID
 			| SYSTEM_P
+			| SYSTEM_USER
 			| TABLE
 			| TABLES
 			| TABLESAMPLE