Add support for fleeting heartbeats/connectivity check before instance acquisition
What does this MR do?
Adds a feature flag FF_USE_FLEETING_ACQUIRE_HEARTBEATS, that when enabled, performs a connectivity check on instances before assigning them a job.
Why was this MR needed?
An instance can disappear after becoming ready.
What's the best way to test this MR?
What are the relevant issue numbers?
Related taskscaler change: Implement acquire heartbeat functionality (gitlab-org/fleeting/taskscaler!58 - merged)
Closes #38271 (closed)