[MDEV-16872] Add CAST(expr AS FLOAT) - Jira

As of version 10.4, MariaDB has CAST(expr AS DOUBLE), but does not have CAST(expr AS FLOAT) yet.

We'll implement CAST(expr AS FLOAT) under terms of this MDEV.

This query:

SELECT CAST(expr AS FLOAT) AS f1 ... ;

will create a column of the FLOAT data type.

blocks

MDEV-16248 Row based replication to spider with float column fails on delete/update

is blocked by

MDEV-19468 Hybrid type expressions return wrong format for FLOAT

is duplicated by

MDEV-7295 cast($expression as float) should work

relates to

MDEV-16873 Compare two FLOAT expressions as floats rather than doubles