Create aip-51.md (#252)

Added the AIP 51 for changing beneficiary addresses

aips/aip-51.md

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+---
+
+aip: 51
+
+title: Changing beneficiaries for operators
+
+author: junkil-park, michelle-aptos, movekevin, wintertoro
+
+discussions-to: https://github.com/aptos-foundation/AIPs/issues/251
+
+Status: In Review
+
+type: Standard Framework
+
+created: 10/3/2023
+
+---
+
+
+# AIP-51 - Changing beneficiaries for operators
+
+## Summary
+
+Currently, the operator commission goes to the operator's address. We have received multiple requests for the ability to set a different beneficiary address. One example use case is when operators want their commission to go to a cold storage wallet while their operator address is used for day-to-day operations. This feature applies to the staking contract, the delegated staking contract, and the vesting contract.
+
+
+### Goals
+
+The goal is to allow operators to set a different beneficiary address for their commission rewards. So, the following scenarios should be possible:
+- An operator wants to store commission rewards in a cold wallet. This change would introduce the decoupling of the wallet and operator keys so that funds can be sent to an address different from the operator's account.
+- The funds would remain in a cold wallet while the operator can perform other operations in a hot wallet.
+
+### Out of Scope
+
+Changing the beneficiaries for token owners (e.g., stakers, delegators) is out of the scope of this AIP.
+
+### Impact
+
+This feature impacts operators and their beneficiaries. If a current or new operator has never set a beneficiary address explicitly, their beneficiary address is the operator address itself by default.
+
+## Specification
+* The beneficiary recieves the commission rewards instead of the operator.
+* The beneficiary of a current operator is the operator address itself.
+* The beneficiary of a new operator is the operator address itself by default.
+* The operator has permission to change its beneficiary address.
+* The beneficiary change takes effect immediately.
+* The beneficiary can request to withdraw the commission rewards.
+* An event is emitted when the beneficiary address is changed.
+
+## Reference Implementation
+Please refer to the [reference implementation](https://github.com/aptos-labs/aptos-core/pull/10455).
+
+## Testing
+- What is the testing plan?
+Automated unit tests and testing in devnet/testnet before releasing to mainnet.
+
+## Risks and Drawbacks
+When the operator address is set to a new address, if the beneficiary address remains in the old one, the commission rewards can go to the wrong beneficiary address. To mitigate this, when the pool owner sets a new operator, the beneficiary of the operator will be set to the operator address by default.
+
+Suppose an operator sets the beneficiary to their separate account for rewards and delegates the operator account and the operation to a third party (e.g., contractor). In that case, there is a risk that the third party can change the beneficiary address to their address. No mitigation for this risk is part of this AIP because it is not a common case and is out of scope. A sophisticated access control mechanism is required to mitigate this risk.
+
+## Timeline
+
+### Suggested deployment timeline
+
+This change will be part of the v1.8 release.
+
+### Security Considerations
+
+The framework change in delegation_pool.move will be audited before it is considered complete.