[CI] Introduce assign-build:getAuthToken
Summary: Generates auth token for use with App Store Connect API: https://developer.apple.com/documentation/appstoreconnectapi/generating_tokens_for_api_requests
Test Plan: Was able to successfully make API calls
Reviewers: ashoat, palys-swm
Reviewed By: ashoat
Subscribers: palys-swm, Adrian, karol-bisztyga, benschac, boristopalov
Differential Revision: https://phabricator.ashoat.com/D2365