slots: slot lenience must take into account block proposal portion (#9138)

* aura, babe: proposal slot lenience must take into account proposal portion

* slots: add support for max_block_proposal_slot_portion

* fix compilation

* slots: add tests

* aura: fix comment

Co-authored-by: Bastian Köcher <[email protected]>

* slots: log the actual proposing duration after lenience is applied

Co-authored-by: Bastian Köcher <[email protected]>