Set Tryton as default user agent header
Sendcloud gives us the partner id 03c1facb-63da-4bb1-889c-192fc91ec4e6 to be set as https://api.sendcloud.dev/docs/sendcloud-public-api/parcels/operations/create-a-parcel#request-headers This way they can recognize Tryton as Sendcloud partner.
Closes #11979 (closed)