Poll the engine_exchangeTransitionConfigurationV1 endpoint (#3047)

## Issue Addressed

There has been an [`engine_exchangetransitionconfigurationv1`](https://github.com/ethereum/execution-apis/blob/main/src/engine/specification.md#engine_exchangetransitionconfigurationv1) method added to the execution API specs.

The `engine_exchangetransitionconfigurationv1` will be polled every 60s as per this PR: https://github.com/ethereum/execution-apis/pull/189. If that PR is merged as-is, then we will be matching the spec. If that PR *is not* merged, we are still fully compatible with the spec, but just doing more than we are required.

## Additional Info

- [x] ~~Blocked on #2837~~
- [x] Add method to EE integration tests

beacon_node/client/src/builder.rs

@@ -723,6 +723,9 @@ where
                 execution_layer.spawn_clean_proposer_preparation_routine::<TSlotClock, TEthSpec>(
                     beacon_chain.slot_clock.clone(),
                 );
+
+                // Spawns a routine that polls the `exchange_transition_configuration` endpoint.
+                execution_layer.spawn_transition_configuration_poll(beacon_chain.spec.clone());
             }
         }