Automatically collect all the migrations as manual listing of migration jobs in collector is not reliable
When creating a migration job, it should be first implemented in `jobs/mongodb_migration/src/mongodb_migration/migrations/` and then manually added to `jobs/mongodb_migration/src/mongodb_migration/collector.py` which is very easy to forget about (see https://github.com/huggingface/datasets-server/pull/1034, https://github.com/huggingface/datasets-server/pull/1043#discussion_r1177606263). 

Would be good to implement automatic creation of a migration collector with all the migrations from `mongodb_migration/migrations` directory.
 
Automatically collect all the migrations as manual listing of migration jobs in collector is not reliable: When creating a migration job, it should be first implemented in `jobs/mongodb_migration/src/mongodb_migration/migrations/` and then manually added to `jobs/mongodb_migration/src/mongodb_migration/collector.py` which is very easy to forget about (see https://github.com/huggingface/datasets-server/pull/1034, https://github.com/huggingface/datasets-server/pull/1043#discussion_r1177606263). 

Would be good to implement automatic creation of a migration collector with all the migrations from `mongodb_migration/migrations` directory.
 