Create AIP for multi-round checkout

[meglister]

User Story:
As a follow up to #1796 , we want to implement multi-round checkout functionality in Grants Stack. In order to do so we will need to make changes to the Allo protocol. This ticket covers discussing and agreeing on the format and timing of those changes with the Allo team. It does not include actually making any changes/commits.

Acceptance Criteria
AIP has been discussed with Allo team and posted to Github

Product & Design Links: None

Tech Details:

Open Questions:

Notes/Assumptions:

• This includes writing test coverage for the functionality added in the ticket, if necessary

[meglister]

@gravityblast we were talking about you picking this up if you're able

[gravityblast]

The implementation of AIP-13 has been merged allo-protocol/allo-contracts#69