initial support for vector indexes

MDEV-33407 Parser support for vector indexes

The syntax is

  create table t1 (... vector index (v) ...);

limitation:
* v is a binary string and NOT NULL
* only one vector index per table
* temporary tables are not supported

MDEV-33404 Engine-independent indexes: subtable method

added support for so-called "high level indexes", they are not visible
to the storage engine, implemented on the sql level. For every such
an index in a table, say, t1, the server implicitly creates a second
table named, like, t1#i#05 (where "05" is the index number in t1).
This table has a fixed structure, no frm, not accessible directly,
doesn't go into the table cache, needs no MDLs.

MDEV-33406 basic optimizer support for k-NN searches

for a query like SELECT ... ORDER BY func() optimizer will use
item_func->part_of_sortkey() to decide what keys can be used
to resolve ORDER BY.

sql/sql_class.h

@@ -462,8 +462,8 @@ private:
 
 class Key :public Sql_alloc, public DDL_options {
 public:
-  enum Keytype { PRIMARY, UNIQUE, MULTIPLE, FULLTEXT, SPATIAL, FOREIGN_KEY,
-                 IGNORE_KEY};
+  enum Keytype { PRIMARY, UNIQUE, MULTIPLE, FULLTEXT, SPATIAL, VECTOR,
+                 FOREIGN_KEY, IGNORE_KEY};
   enum Keytype type;
   KEY_CREATE_INFO key_create_info;
   List<Key_part_spec> columns;