Add support for multiple versions of an extension and ALTER EXTENSION UPDATE.

This follows recent discussions, so it's quite a bit different from
Dimitri's original.  There will probably be more changes once we get a bit
of experience with it, but let's get it in and start playing with it.

This is still just core code.  I'll start converting contrib modules
shortly.

Dimitri Fontaine and Tom Lane

src/include/nodes/parsenodes.h

@@ -1546,6 +1546,14 @@ typedef struct CreateExtensionStmt
 	List	   *options;		/* List of DefElem nodes */
 } CreateExtensionStmt;
 
+/* Only used for ALTER EXTENSION UPDATE; later might need an action field */
+typedef struct AlterExtensionStmt
+{
+	NodeTag		type;
+	char	   *extname;
+	List	   *options;		/* List of DefElem nodes */
+} AlterExtensionStmt;
+
 typedef struct AlterExtensionContentsStmt
 {
 	NodeTag		type;