Add json extract function for my sql

[ssanchez91]

The JSON_UNQUOTE function in MySQL is used to remove the surrounding double quotes from a JSON string or a JSON value that is stored as a string. It is particularly useful when working with JSON data in a MySQL database, as it helps to extract the raw value from the JSON string, making it easier to use and compare in queries and other operations. The function can be used with both static JSON strings and parameterized values, providing flexibility when working with JSON data.

[DaDeather]

@ssanchez91 This is not quite complete though. There's another PR also open for this case https://github.com/beberlei/DoctrineExtensions/pull/391 where the implementation includes every functionality of the JSON_EXTRACT method.