incompatible_remote_downloader_send_all_headers

[Yannic]

Description: This flag changes the JSON sent to the gRPC remote downloader as qualifier bazel.auth_headers from a map of strings to a map of list of strings.

Rationale: Headers can, in general, be defined multiple times, not just once.

Migration: Flipping this flag might cause failures when using the Remote Assets API when the server supports reading this qualifier. Contact your service provider for more information.

[Yannic]

cc @tjgq

Please add the following labels: breaking-change-6.0, incompatible-change

[meteorcloudy]

Please don't forget adding migration-ready label, otherwise the incompatible flags pipeline won't test this one.