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_handler.cc

@@ -668,6 +668,7 @@ mysql_ha_fix_cond_and_key(SQL_HANDLER *handler,
 
       if (c_key->algorithm == HA_KEY_ALG_RTREE ||
           c_key->algorithm == HA_KEY_ALG_FULLTEXT ||
+          c_key->algorithm == HA_KEY_ALG_VECTOR ||
           (ha_rkey_mode != HA_READ_KEY_EXACT &&
            (table->key_info[handler->keyno].index_flags &
             (HA_READ_NEXT | HA_READ_PREV | HA_READ_RANGE)) == 0))