Adds multi-block election types and refactors current pallets to support new interfaces and types

[gpestana]

This PR refactors the types and structs required to run a mulit-block election and updates the EPM, staking-pallet and all dependent pallets to use the multi-block types. The Westend runtime is configured to run a 1 paged election, which is a noop refactor compared to the current single-block election.

Notable changes since last reviews:

• EPM-MB: Supports only single page target snapshot #6414
• Adds max backers per winner bound trimming in the phragmen implementation #6482

Tasks based on feedback that can be closed after merging this PR: Umbrella ticket for multi-block election tasks to improve after PR#6034.

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The multi-block election provider pallet is wip and it's added in a separate PR (#6213).

To-do before merging:

• test with chopsticks/follow-chain
• add benchmarks for on_intialize in pallet staking

[Ank4n]

Only looked at traits. Will do another pass.

[kianenigma]

Approving tentatively that the direction is corrected and checked in previous rounds, and audit can already begin.

[paritytech-workflow-stopper]

All GitHub workflows were cancelled due to failure one of the required jobs.
Failed workflow url: https://github.com/paritytech/polkadot-sdk/actions/runs/12805885696
Failed job name: build-rustdoc