serve: Start assignment worker as sync operation

Previously partition assignment worker was configured to run only when raft is enabled. But because partition assignment is now prerequisite for partition backups as well, we need to run this migration even if raft is disabled.

Also currently, partition assignment worker is only creating partition assignment keys in the badger DB, however there is also repository key within partition's KV that needs to be updated to be able to find all the relative paths within a partition. This is needed to include all the repositories in a partition level snapshot.

Closes #6894 (closed)