Add support for character set prefixes on string literals

[lukaseder]

MySQL has a syntax to provide a character set with a string literal:

• https://dev.mysql.com/doc/refman/8.0/en/charset-literal.html

E.g.

SELECT _latin1'abc';

We should:

• Support this in the parser
• Support this in our query object model, similar to the COLLATE clause (this needs further investigation)