Make pallet_referenda::TracksInfo::tracks return type more flexible

[olanod]

At the moment the return type of pallet_referenda::TracksInfo::tracks is a slice of tracks with a static lifetime, the trait is not very flexible as it only allows simple implementations where the tracks have to be hard-coded, relaxing this signature can open up new use cases like updating existing tracks without runtime upgrades(to avoid this) or a "dao pallet" that can create tracks on the fly and leverage the existing infrastructure provided by referenda.

In PR #2072 I propose returning an Iterator of Cow<'static, Track<_, _, _>>, it keeps compatibility with existing implementations that might want to keep defining tracks statically for performance reasons, it allows returning owned tracks that might come from the chain storage or a combination of both. An alternative of returning a Vec<Track>> is not ideal as it would still restrict us to less performant and more memory hungry implementations.

[gavofyork]

Sounds sensible. As long as it doesn't impact performance for the base case not add substantial additional logic then I'm in favour.

[Polkadot-Forum]

This issue has been mentioned on Polkadot Forum. There might be relevant details there:

https://forum.polkadot.network/t/opengov-for-everyone-1-ksm-1-dao/7781/1