Extending VestingWallet to have revocability

🧐 Motivation
The existing implementation for VestingWallet provides a trustless base contract which is correct as a default. However, vesting agreements in real world applications may require the ability to cancel vesting in cases where agreements between the owner and beneficiary falls apart. An example use case can be when employees leave or get removed from an organization.

There have been a prior implementation of this feature in issue #1416 however it appears to be deprecated.

📝 Details
The extension I would like to implement is VestingWalletRevocable, it provides owner of VestingWallet to make cancellations with the following constraints:

• Revocation only returns unvested tokens to the owner, while vested tokens remain claimable.
• Once a contract has been revoked, it cannot be reinstated.

Steps

• Tracking revocation states (prevent double revoking)
• Implement revoke() which transfers unvested tokens back to owner
• Override existing vestedAmount() to the use original token allocation amount to account for the revoked token affecting the balance and the vesting schedule.

Let me know your thoughts and if this aligns with the project.

Start with the existing VestingWallet implementation and review the prior approach in issue #1416. Define the revocation state and behavior around unvested-token returns, claimability of vested tokens, and irreversible cancellation; done means the proposed VestingWalletRevocable extension satisfies all listed constraints.