dynamic sql example based on dialect

[tanner0101]

In Fluent 4, it should be possible to use custom types that can change dynamically based on dialect. For example:

struct CustomTextType: SQLExpression {
    func serialize(to serializer: SQLSerializer) {
        switch serializer.dialect {
        case .mysql: serializer.write("VARCHAR(64)")
        default: serializer.write("TEXT")
        }
    }
}

struct User: Model {
    let name = Field<String>("name", dataType: .sql(CustomTextType()))
    ...
}

We should have a test case to ensure this works and document it.