`Consideration` trait generic over `Footprint` and indicates zero cost
for a give footprint.

`Consideration` trait is generic over `Footprint` (currently defined
over the type with the same name). This makes it possible to setup a
custom footprint (e.g. current number of proposals in the storage).

`Consideration::new` and `Consideration::update` return an
`Option<Self>` instead `Self`, this make it possible to indicate a no
cost for a specific footprint (e.g. if current number of proposals in
the storage < max_proposal_count / 2 then no cost).

These cases need to be handled for
https://github.com/paritytech/polkadot-sdk/pull/3151