Add PG::JSONB converter

[vladfaust]

# json:
Bytes[123, 34, 102]
# jsonb:
Bytes[1, 123, 34, 102, 111]

Thanks @alecdwm for bugreport!

[vladfaust]

Hey @alecdwm could you test the latest development branch commit please? I did some refactoring which should not cause any conflicts — in theory 😅

[vladfaust]

Closed by ecf58ca.

[alecdwm]

Seems to work to me, thanks!