Adding try_state hook for Vesting pallet

[Doordashcon]

Part of #239.

The invariant:

• At the blocks before vesting begins, the locked amount of a vesting schedule must be equal to the product of the duration and the release per block amount when the locked amount is divisible by the per block amount, else the final vesting block should be equal to the unvested amount. This should also hold true during vesting schedules.

polkadot address: 12zsKEDVcHpKEWb99iFt3xrTCQQXZMu477nJQsTBBrof5k2h

[gpestana]

Looks in the right direction, I think it would be useful to do a small refactoring on the externalities builder to make it easier to run the try state checks at the end of the tests.

Also, it would be easier for the reviewers if the feature branch is in the polkadot-sdk repo and not on a fork of yours, so we can easily pull it and test it locally.

[ggwpez]

/tip medium

(Tip for both this one and #1871)

[substrate-tip-bot]

@Doordashcon Contributor did not properly post their account address.

Make sure the pull request description (or user bio) has: "{network} address: {address}".

[ggwpez]

/tip medium

[substrate-tip-bot]

@ggwpez Invalid tip size. Please specify one of small, medium, large.

[ggwpez]

/tip medium

[substrate-tip-bot]

@ggwpez A medium (80 DOT) tip was successfully submitted for @Doordashcon (12zsKEDVcHpKEWb99iFt3xrTCQQXZMu477nJQsTBBrof5k2h on polkadot).

https://polkadot.js.org/apps/?rpc=wss%3A%2F%2Frpc.polkadot.io#/referenda

[Polkadot-Forum]

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

https://forum.polkadot.network/t/polkadot-digest-25-jan-2024/5837/1

[paritytech-cicd-pr]

The CI pipeline was cancelled due to failure one of the required jobs.
Job name: cargo-clippy
Logs: https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/5114781

[Doordashcon]

@ggwpez @gpestana please review 🙏