Allow multiple artifact URLs to be configured for a single policy

[ycombinator]

Taken from elastic/fleet-server#2586:

Describe the enhancement:
Today you can configure one or more artifact repositories for air gapped networks to serve downloads but you can only assign one download URI per policy. The enhancement would be like that of the output configurations in Kibana -> Fleet -> Settings where you can then set many URIs for the Elastic agent to download updates from.

Describe a specific use case for the enhancement or feature:
The use case is that we prefer agents to use our hosted downloads but if they are unavailable to from 1 source they can try another source such as another server in the network or from the default Elastic artifacts repo. I understand a load balancer could handle this but something built in would reduce complexity.

What is the definition of done?

• Once Fleet Server is capable of sending Agent multiple URIs, Agent tries them one at a time, in order.

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)