Add external_sku to Adjustment

[joannasese]

To fetch external_sku:

charge = recurly.Adjustment(
  unit_amount_in_cents=5000,
  currency='USD',
  item_code='cardigan_bushwick',
)
account.charge(charge)

for adjustment in account.adjustments():
    print('Adjustment: %s' % adjustment.external_sku)

[bhelx]

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