Add support for JSON columns

[nguyenk]

It would be very nice if JSON columns return a stdClass instead of a string (a bit like datetime SQL type already returns a DateTimeImmutable PHP Type)

[moufmouf]

We need to wait support for JSON datatype in DBAL MySQL driver.
As soon as it is available, we are good to go.

See:
doctrine/dbal#2455
doctrine/dbal#2266

[moufmouf]

doctrine/dbal#2455 and doctrine/dbal#2266 are now closed. Will be shipped in Doctrine 2.6.
TODO: let's test this when Doctrine DBAL 2.6 is tagged.