Make get_extension_schema() available

This routine is able to retrieve the OID of the schema used with an
extension (pg_extension.extnamespace), or InvalidOid if this information
is not available.  plpgsql_check embeds a copy of this code when
performing checks on functions, as one out-of-core example.

Author: Pavel Stehule
Reviewed-by: Julien Rouhaud
Discussion: https://postgr.es/m/CAFj8pRD+9x55hjDoi285jCcjPc8uuY_D+FLn5RpXggdz+4O2sQ@mail.gmail.com

Author: michaelpq

src/backend/commands/extension.c

@@ -222,7 +222,7 @@ get_extension_name(Oid ext_oid)
  *
  * Returns InvalidOid if no such extension.
  */
-static Oid
+Oid
 get_extension_schema(Oid ext_oid)
 {
 	Oid			result;

src/include/commands/extension.h

@@ -47,6 +47,7 @@ extern ObjectAddress ExecAlterExtensionContentsStmt(AlterExtensionContentsStmt *
 
 extern Oid	get_extension_oid(const char *extname, bool missing_ok);
 extern char *get_extension_name(Oid ext_oid);
+extern Oid	get_extension_schema(Oid ext_oid);
 extern bool extension_file_exists(const char *extensionName);
 
 extern ObjectAddress AlterExtensionNamespace(const char *extensionName, const char *newschema,