Relax shard_transfer_mode=auto to work with non-primary key / unique indexes [PG 16]

Now that [89e46da5e511a6970e26a020f265c9fb4b72b1d2](https://github.com/postgres/postgres/commit/89e46da5e511a6970e26a020f265c9fb4b72b1d2) is merged to Postgres 16, we should consider doing `auto` shard moves without requiring primary key / unique index.

- Phase-1 could be use any existing index on the tables
- Phase-2 could be to create an index on-the-fly if there are no suitable indexes