Refactor: Change `ALTER FRAGMENT SET RATE_LIMIT` to `ALTER FRAGMENT SET (BACKFILL_RATE_LIMIT | SOURCE_RATE_LIMIT | DML_RATE_LIMIT | SINK_RATE_LIMIT)`

As per title. Currently we just lump it into a single alter fragment set rate limit, however if a fragment has both sink rate limit and backfill rate limit, the semantics of alter becomes ambiguous. It's better to decouple these.