## Description 

Add `BridgeAuthorityAggregator`.
1. PR is big because 70% are tests.
2. Meat is in `bridge_authority_aggregator.rs (200+ lines excluding
tests) ` and `types.rs (impl
CommitteeTrait<BridgeAuthorityPublicKeyBytes> for BridgeCommittee)`


## Test Plan 
unit tests


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

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

### Release notes
Add `BridgeAuthorityAggregator`.