Add proper support for timestamp arrays

[0xTim]

[fabianfett]

Do we want to add this support also for the potentially new PSQLCodable type? You would add this by adding

extension Date: PSQLArrayElement {
    static var psqlArrayType: PSQLDataType { .timestamptzArray }
    static var psqlArrayElementType: PSQLDataType { .timestamptz }
}

to Array+PSQLCodable. This should be it!

[0xTim]

Yeah if that's simpler and better long term we should migrate to that

[gwynne]

Superseded (at last) by #463