Update event payloads

[NothingButThyme]

Description

The purpose of this PR is to clearly specify the fields that will be expected in messages coming from the Vendor, and to provide a starting point for discussion about changes to those fields.

Also includes some updates that have been discussed previously:

1. iFrameRedirect event
2. Added retailPrice to the AddToCart Metadata

Type of Change

• [ ] Bug Fix
• [X ] New Feature
• [ ] Breaking Change
• [ ] Refactor
• [ ] Documentation
• [ ] Other (please describe)

Checklist

• [ ] I have read the contributing guidelines
• [ ] Existing issues have been referenced (where applicable)
• [ ] I have verified this change is not present in other open pull requests
• [ ] Functionality is documented
• [ ] All code style checks pass
• [ ] New code contribution is covered by automated tests
• [ ] All new and existing tests pass

[sdashner]

We will need 2 net new events for atc and continue to cart flow

Vendor comms

• continue to cart would have {message, EventPayload} client comms
• add to cart status event {success: boolean, message: string, projectId: string, versionId }