[Dart][Client] Support parsing DateTime

[kalymero]

I've got an issue while parsing an Array of Strings in date-time format

        content:
            application/json:
              schema:
                type: array
                items:
                  type: string
                  format: date-time

which the generator translates into a List<DateTime>.
When the List is unwrapped and then it tries to _deserialize the DateTime it does not find any suitable parser.

This just fixes it

@ahmednfwela

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[kuhnroyal]

cc @agilob

[wing328]

Samples updated via b39a1f8