sql: introduce <CREATE CAST> statement

[Korablev77]

As a part of introducing advanced strict typing, we may add <CREATE CAST> statement, which determines or overrides types casting. It can be useful for users who get used to behaviour demonstrated by other DBs.

Syntax according to ANSI (11.63 <user-defined cast definition>):

<user-defined cast definition> ::=
CREATE CAST <left paren> <source data type> AS <target data type> <right paren> WITH <cast function> [ AS ASSIGNMENT ]
<cast function> ::= <specific routine designator>
<source data type> ::= <data type>
<target data type> ::= <data type>