Enforce EL RPC method types in the RPC method resolver

[mehdi-aouadi]

Currently the json RPC methods extending the abstract AbstractEngineJsonRpcMethod used by the ExecutionClientHandler might have different type parameter from the one supplied to the EngineJsonRpcMethodsResolver::getMethod which could lead to class cast exceptions.
Example:

DummyRpcMethodClass extends AbstractEngineJsonRpcMethod<ReturnTypeClass>

ExecutionClientHandlerImpl {
...
@Override
  public SafeFuture<BadReturnTypeClass> dummyRpcMethod(
    return engineMethodsResolver
        .getMethod(
            EngineApiMethod.ENGINE_DUMMY_RPC_METHOD,
            () -> spec.atSlot(slot).getMilestone(),
            BadReturnTypeClass.class)
        .execute(paramsBuilder.build());
  }
...
}

In the example above, the dummyRpcMethod shoudl have ReturnTypeClass instead of BadReturnTypeClass.
We could make the EngineJsonRpcMethodsResolver detect the RPC method return type dynamically.
Possibles approaches could be to define the return type in the method itself or make use of reflection to dynamically determine it...

[mehdi-aouadi]

A better approach I've found is to refactor the unit tests and make them check the return type of the execution engine's response futures.