This PR replaces `pallet-validator-set` with a new
`pallet-external-validators` pallet from Tanssi.
## Key Changes
- **New ExternalValidators Pallet**
- Supports whitelisted validators (set by governance, not rewarded)
- Manages external validators (can be enabled/disabled)
- Implements era-based rotation (eras change after configurable
sessions)
- **Bridge Integration**
- Updated `EigenLayerMessageProcessor` in
`operator/primitives/bridge/src/lib.rs`
- Replaced the old `SetValidators` command with
```rust
ReceiveValidators { validators, external_index }
```
