update `overseer::Seconded`

## Issue summary

Needed updates from the current code:
- Notify collation seconded (https://github.com/ChainSafe/gossamer/issues/4675)
- See if we've unblocked other collations for seconding.
- Dequeue next collation and fetch if prospective-candidate exists.

## Links

- https://github.com/paritytech/polkadot-sdk/blob/3ff1b1db36260cbc47297ab753e2dcec1f5999fd/polkadot/node/network/collator-protocol/src/validator_side/mod.rs#L1490

Start by locating the overseer::Seconded implementation in gossamer and compare its behavior with the linked polkadot-sdk validator_side/mod.rs reference. Trace the collation-seconding flow and verify that collation-seconded notification, newly unblocked collations, and prospective-candidate dequeue and fetch behavior are covered.

Written by the indexing model from the issue text.