Add dialect param to use CHAR instead of TEXT for Utf8 unparsing for MySQL

[sgrebnov]

Which issue does this PR close?

Fixes spiceai/spiceai#1948

Rationale for this change

MySQL cast function does not support TEXT and requires CHAR (automatically returns VARCHAR, TEXT, LONGTEXT)

CHAR[(N)] [charset_info]
Produces a string with the VARCHAR data type, unless the expression expr is empty (zero length), in which case the result type is CHAR(0). If the optional length N is given, CHAR(N) causes the cast to use no more than N characters of the argument. No padding occurs for values shorter than N characters. If the optional length N is not given, MySQL calculates the maximum length from the expression. If the supplied or calculated length is greater than an internal threshold, the result type is TEXT. If the length is still too long, the result type is LONGTEXT.

Example query:

 select * from customer where c_custkey = 'building

Before this change (fails in MySQL)

SELECT `customer`.`c_custkey`, ... FROM `customer` WHERE (CAST(`customer`.`c_custkey` AS TEXT) = 'building')

After this change (works in MySQL)

SELECT `customer`.`c_custkey`,  ... FROM `customer` WHERE (CAST(`customer`.`c_custkey` AS CHAR) = 'building')