[1 / 5] Optimize logic for gossiping assignments (#4848)

This is part of the work to further optimize the approval subsystems, if
you want to understand the full context start with reading
https://github.com/paritytech/polkadot-sdk/pull/4849#issue-2364261568,
however that's not necessary, as this change is self-contained and nodes
would benefit from it regardless of subsequent changes landing or not.

While testing with 1000 validators I found out that the logic for
determining the validators an assignment should be gossiped to is taking
a lot of time, because it always iterated through all the peers, to
determine which are X and Y neighbours and to which we should randomly
gossip(4 samples).

This could be actually optimised, so we don't have to iterate through
all peers for each new assignment, by fetching the list of X and Y peer
ids from the topology first and then stopping the loop once we took the
4 random samples.

With this improvements we reduce the total CPU time spent in
approval-distribution with 15% on networks with 500 validators and 20%
on networks with 1000 validators.

## Test coverage:

`propagates_assignments_along_unshared_dimension` and
`propagates_locally_generated_assignment_to_both_dimensions` cover
already logic and they passed, confirm that there is no breaking change.

Additionally, the approval voting benchmark measure the traffic sent to
other peers, so I confirmed that for various network size there is no
difference in the size of the traffic sent to other peers.

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Signed-off-by: Alexandru Gheorghe <[email protected]>