get https://darwin.v7labs.com/api/v2/teams//items//slots//stream//sign
Returns pre-signed m3u8 index for streaming the requested slot
with specified quality preset.
'high' quality preset is default, i.e. the same as ommiting this parameter entirely.
Other quality presets are only available if the Item.slots[n].available_quality_presets
field lists them for given item/slot.
Callback