Add EmitWarningsOnPlaceholders("age") (apache#2000)

Added EmitWarningsOnPlaceholders("age") to warn when an undefined GUC parameter is set for a placeholder named "age".
Warnings are issued when PostgreSQL starts and when SET statements are executed.

When PostgreSQL starts
```
2024-08-01 18:32:31.220 JST [2039799] WARNING: invalid configuration parameter name "age.invalid_parameter", removing it
```

When SET statement is executed
```
ERROR: invalid configuration parameter name "age.invalid_parameter"
DETAIL: "age" is a reserved prefix.
```

Regression tests were added.

regress/expected/name_validation.out

@@ -475,6 +475,14 @@ NOTICE:  graph "graph123" has been dropped
 
 (1 row)
 
+--
+-- Test GUC names
+--
+SET age.enable_containment TO ON;
+SET age.invalid_parameter TO ON;
+ERROR:  invalid configuration parameter name "age.invalid_parameter"
+DETAIL:  "age" is a reserved prefix.
+SET any_placeholder.any_parameter TO ON;
 --
 -- End of test
 --

regress/sql/name_validation.sql

@@ -178,6 +178,13 @@ SELECT create_elabel('graph123', 'elabel0123456789012345678901234567890123456789
 -- clean up
 SELECT drop_graph('graph123', true);
 
+--
+-- Test GUC names
+--
+SET age.enable_containment TO ON;
+SET age.invalid_parameter TO ON;
+SET any_placeholder.any_parameter TO ON;
+
 --
 -- End of test
 --

src/backend/utils/ag_guc.c

@@ -42,4 +42,5 @@ void define_config_params(void)
                              NULL,
                              NULL,
                              NULL);
+    EmitWarningsOnPlaceholders("age");
 }