[FIX] base: avoid creating a new index at each model initialization

As example, on a runbot database, there are 13 duplicated indexes

```
4362411-14-0-all=> \d res_users_apikeys;
                                         Table "public.res_users_apikeys"
   Column    |            Type             | Collation | Nullable |                    Default
-------------+-----------------------------+-----------+----------+-----------------------------------------------
 id          | integer                     |           | not null | nextval('res_users_apikeys_id_seq'::regclass)
 name        | character varying           |           | not null |
 user_id     | integer                     |           | not null |
 scope       | character varying           |           |          |
 index       | character varying(8)        |           |          |
 key         | character varying           |           |          |
 create_date | timestamp without time zone |           |          | timezone('utc'::text, now())
Indexes:
    "res_users_apikeys_pkey" PRIMARY KEY, btree (id)
    "res_users_apikeys_user_id_index_idx" btree (user_id, index)
    "res_users_apikeys_user_id_index_idx1" btree (user_id, index)
    "res_users_apikeys_user_id_index_idx10" btree (user_id, index)
    "res_users_apikeys_user_id_index_idx11" btree (user_id, index)
    "res_users_apikeys_user_id_index_idx12" btree (user_id, index)
    "res_users_apikeys_user_id_index_idx2" btree (user_id, index)
    "res_users_apikeys_user_id_index_idx3" btree (user_id, index)
    "res_users_apikeys_user_id_index_idx4" btree (user_id, index)
    "res_users_apikeys_user_id_index_idx5" btree (user_id, index)
    "res_users_apikeys_user_id_index_idx6" btree (user_id, index)
    "res_users_apikeys_user_id_index_idx7" btree (user_id, index)
    "res_users_apikeys_user_id_index_idx8" btree (user_id, index)
    "res_users_apikeys_user_id_index_idx9" btree (user_id, index)
Check constraints:
    "res_users_apikeys_index_check" CHECK (char_length(index::text) = 8)
Foreign-key constraints:
    "res_users_apikeys_user_id_fkey" FOREIGN KEY (user_id) REFERENCES res_users(id)
```

closes odoo/odoo#58746

X-original-commit: 94068037fbddc816926af7c68c19780926832722
Signed-off-by: Christophe Simonis <chs@odoo.com>

odoo/addons/base/models/res_users.py

@@ -1614,7 +1614,7 @@ class APIKeys(models.Model):
             key varchar not null,
             create_date timestamp without time zone DEFAULT (now() at time zone 'utc')
         );
-        CREATE INDEX ON {table} (user_id, index);
+        CREATE INDEX IF NOT EXISTS res_users_apikeys_user_id_index_idx ON {table} (user_id, index);
         """.format(table=self._table, index_size=INDEX_SIZE))
 
     @check_identity