feat: new leader schedule by vote delegation (SIMD-0180)

[jstarry]

Problem

SIMD-0180 proposes a new change to the leader schedule algorithm to use vote account delegations rather than total node delegations.

Summary of Changes

• Switch over to the new leader schedule algorithm using a feature gate. The new leader schedule algorithm will be applied two epochs after the feature gate activation epoch.

• A new vote_keyed::LeaderSchedule struct was added which also implements the LeaderScheduleVariant trait.

Fixes #
Feature Gate Issue: #4573

[joncinque]

This PR contains changes to the solana sdk, which will be moved to a new repo within the next week, when v2.2 is branched from master.

Please merge or close this PR as soon as possible, or re-create the sdk changes when the new repository is ready at https://github.com/anza-xyz/solana-sdk

[AshwinSekar]

Logic looks correct, I like exposing slot_leaders so we don't have to perform an additional lookup

[jstarry]

Thanks for the review @AshwinSekar, I'll rebase once it's clear where this new feature gate key should be added now that sdk is in a separate repo

[t-nelson]

can we ref the simd in the title?

[jstarry]

@t-nelson @AshwinSekar this is ready for another review pass. I've rebased on master to pick up the new leader schedule trait introduced in #4973.

[AshwinSekar]

Logic looks correct, will wait for trent's approval

[t-nelson]

@mergify rebase

[mergify]

rebase

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