A0-2671: Add try-state hook to the elections pallet

[pmikolajczyk41]

Description

This is the first PR introducing try-state hook to our pallets. Hopefully, more to come.

Why?

try-state is a similar concept to try-runtime. Basically, we can put all (storage) invariant checks into a hook and trigger them from anywhere we need. By default, they can be called from most of try-runtime commands, like follow-chain. More info: https://forum.polkadot.network/t/testing-complex-frame-pallets-discussion-tools/356#try-runtime-follow-chain-trystate-4

Local testing

I have broken some of the checks in ensure_validators_are_ok function and then:

$ cargo build --release --features try-runtime
$ ./scripts/run_nodes.sh -b false
$ ./target/release/aleph-node try-runtime --runtime existing --chain /tmp/chainspec.json follow-chain --uri ws://localhost:9944 --keep-connection

Note: Cardinal-Cryptography/substrate#39 is required for this scenario.

Type of change

• New feature (non-breaking change which adds functionality)