[BIP-283] Change veBAL USD fee component to the new bb-a-usd that uses Aave V3

TL;DR

This proposal changes the USD portion of fees paid out to veBAL lockers from the old version of bb-a-usd that uses Aave v2 and is in recovery mode to a new pool using Aave v3 and a updated version of the ComposableStableSwap pool.

The choice of bb-a-usd V3(0xfebb0bbf162e64fb9d0dfe186e517d84c395f016) as the new fee token was made by the community here: https://forum.balancer.fi/t/rfc-change-vebal-usd-yield-component/4692#looking-for-input-8.

The change be implemented 3 weeks after the passing of this BIP (May 31st). Lockers will notice the starting on the Wednesday following the first Fee Processing round after this date (June 14th)

Note to integrators

This change may require integrators building on top of veBAL, or derivatives of it to make changes. We are trying to reach out to all integrators and inform them of this change, and will ensure that no changes to fee tokens paid out to claim will occur before June 1st. Please drop a comment here if you have concerns or would like to talk to us about handling this change.

Background and Context

Fee Handling at Balancer

veBAL lockers are entitled to 65% of the protocol fees collected by the Balancer Protocol.

For pools designated as Core Pools these fees are paid out in the form of USDC incentives placed on Hidden Hand which can be earned by veBAL lockers by voting on the pools that generate these fees.

For other pools (no yield bearing component or exceptions for ve8020), these fees are paid out as claimable rewards for veBAL lockers. In this case, all BAL is paid out as BAL, and all other tokens are currently sold for the old bb-a-usd and paid out as such.

Retirement of bb-a-usd

The bb-a-usd token currently paid out to veBAL lockers as protocol fees has been deprecated due to the following reasons:

• Aave has moved to a v3 market, and this pool is based on V2
• This pool is in recovery mode since February due to this Reentrecy Vulnerability.
  • Only proportional deposits and withdraws. (gas heavy)
  • No protocol fees taken/no longer core pool.

Selection of the new Fee token

The new fee token was selected based on community feedback taken in this RFC: https://forum.balancer.fi/t/rfc-change-vebal-usd-yield-component/4692#looking-for-input-8

21 voters participated in a Poll that ran over a period of 1 week. Options included boosted and non-boosted versions of USD and ETH based pools. The new bb-a-usd running on Aave v3 won with 66% of the vote, the next option wstETH/ETH had 33% of the votes. There were no comments in long text form.

Specifications

If this BIP passes. The following changes to the system will occur to enact the change:

• No on-chain changes to Balancer contracts is required to facilitate this change.
• OpCo will adapt the UI to allow claiming and display of the new fee token.
• Mimic will be instructed to change the Smart Vault for fee handling currently under development to handle the new fee token.
• The script currently used to process fees by the Maxi's will be changed to output bb-a-usd v3 the new fee token paid to veBAL assuming the mimic vault is not yet in operation on Mainnet.

The change be implemented on or around 3 weeks after the passing of this BIP (May 31st). Lockers will notice the starting on the Wednesday following the first Fee Processing round after this date (Midnight GMT on June 15th)