## Description 

Uses `Envelope` + `trait Message` for CheckpointSummaries.

## Test Plan 

`cargo simtest`

---
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### Type of Change (Check all that apply)

- [ ] user-visible impact
- [ ] breaking change for a client SDKs
- [x] breaking change for FNs (FN binary must upgrade)
- [x] breaking change for validators or node operators (must upgrade
binaries)
- [ ] breaking change for on-chain data layout
- [x] necessitate either a data wipe or data migration

### Release notes