[Ownable] Add a three-step approach in ownership transfer (Transfer, Accept and ApproveTransfer)

[sprtd]

🧐 Motivation

The current two-step ownership transfer approach does not completely solve the issues surrounding secure transfer of ownership given that the new owner first has to simply call accept_ownership to finalize the transfer.

📝 Details

There is need to add an extra step to further secure the ownership transfer process such that in the event the first owner mistakenly transfers ownership to a malicious actor, ownership is not at risk even when such a bad actor immediately calls accept_ownership. The final transfer of ownership should be majorly determined by the first owner who must call an approve_ownership_transfer function to securely authorize the ownership transfer to the intended entity.

I'm proposing a three-step process:

• 1. Transfer Ownership: transfer_ownership initiates the three-step ownership transfer process by setting the pending owner.

 /// Starts the three-step ownership transfer process by setting the pending owner.
 /// Can only be called by the intended first owner.
 fn transfer_ownership(
       ref self: ComponentState<TContractState>, new_owner: ContractAddress
  ) {
        self.assert_only_owner();
        self._propose_owner(new_owner);
  }

• 2. Accept Ownership: this is 2nd step which involves the ownership acceptance by the pending owner. to prevent name collision with the current accept_ownership of the 2-step approach, we may change the name as consensus can be reached by the community on this.

  /// this is 2nd step which involves the ownership acceptance by the pending owner.
   /// Can only be called by the intended pending owner.
fn consent_to_ownership(ref self: ComponentState<TContractState>) {
      let caller = get_caller_address();
      let pending_owner = self.Ownable_pending_owner.read();
      assert(caller == pending_owner, Errors::NOT_PENDING_OWNER);
      self._consent_to_ownership();
 }

• 3. ApproveOwnershipTransfer: approve_ownership_transfer finishes the ownership transfer process and this can only be called by the first owner to validate the transfer

  /// completes the three-step ownership transfer process by authorizing the ownership transfer to the intended pending owner.
  /// Can only be called by the first owner.
  fn approve_ownership_transfer(
       ref self: ComponentState<TContractState>, new_owner: ContractAddress
   ) {
        self.assert_only_owner();
        // assert that the pending owner has consented
        assert(self._is_pending_ownership_consented(new_owner) == true, Errors::NOT_CONSENTED);
        self._approve_ownership_transfer();
  }