Beats should use the proxy queue when the shipper output is active

[faec]

#34377 adds a proxy queue to prevent the double-allocation issue where events are queued in both Beats and the shipper while waiting on upstream acknowledgment. We need to actually activate that queue when the shipper output is selected; this might require changes to the Beats initialization process, or to handling of agent policies / config updates.

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)