Initial implementation of validator disabling strategy

[tdimitrov]

The strategy is described in this issue comment. This PR outlines an initial implementation with todos.

Related to https://github.com/paritytech/polkadot-sdk/issues/784

Things that needs to be addressed:

• [ ] Fix the TODOs in the code.
• [ ] Add tests
• [ ] Add runtime migration for staking palet
• [ ] Add benchamarks

[paritytech-cicd-pr]

The CI pipeline was cancelled due to failure one of the required jobs. Job name: cargo-check-benches Logs: https://gitlab.parity.io/parity/mirrors/substrate/-/jobs/3306254