refactor(resharding): migrate copy_data to the tasks api

[meskill]

Refactor table copies logic to the api/tasks.

• move out logic for copying from orchestrator/publisher/table to separate data_sync entities
• extend the info types in pgdog-stats for data copy
• migrate to tasks context that is sent to ee
• add WorkerPool entity to distribute load between resharding replicas more evenly - use work stealing with cap configured with resharding_parallel_copies for single replica. Previously the distribution was not fair and some of the replicas could acquire significantly more work
• add more tests coverage for the process.

I verified the copy_data benchmarks and didn't notice any performance changes after the migration. (the benchmarks doesn't cover multiple replicas though, but I think that should improve performance)