Support CREATE ACCESS METHOD

This enables external code to create access methods.  This is useful so
that extensions can add their own access methods which can be formally
tracked for dependencies, so that DROP operates correctly.  Also, having
explicit support makes pg_dump work correctly.

Currently only index AMs are supported, but we expect different types to
be added in the future.

Authors: Alexander Korotkov, Petr Jelínek
Reviewed-By: Teodor Sigaev, Petr Jelínek, Jim Nasby
Commitfest-URL: https://commitfest.postgresql.org/9/353/
Discussion: https://www.postgresql.org/message-id/CAPpHfdsXwZmojm6Dx+TJnpYk27kT4o7Ri6X_4OSWcByu1Rm+VA@mail.gmail.com

src/include/nodes/parsenodes.h

@@ -1379,6 +1379,7 @@ typedef struct SetOperationStmt
 
 typedef enum ObjectType
 {
+	OBJECT_ACCESS_METHOD,
 	OBJECT_AGGREGATE,
 	OBJECT_AMOP,
 	OBJECT_AMPROC,
@@ -2070,6 +2071,18 @@ typedef struct AlterPolicyStmt
 	Node	   *with_check;		/* the policy's WITH CHECK condition. */
 } AlterPolicyStmt;
 
+/*----------------------
+ *		Create ACCESS METHOD Statement
+ *----------------------
+ */
+typedef struct CreateAmStmt
+{
+	NodeTag		type;
+	char	   *amname;			/* access method name */
+	List	   *handler_name;	/* handler function name */
+	char		amtype;			/* type of access method */
+} CreateAmStmt;
+
 /* ----------------------
  *		Create TRIGGER Statement
  * ----------------------