Refactor elcontracts.ELChainWriter.register_as_operator

[ricomateo]

The register_as_operator method in the ELChainWriter claims that it registers the operator given by the operator parameter on EigenLayer.

However this method calls the function registerAsOperator from the DelegationManager under the hood, which in fact registers the caller of the function as an operator on EigenLayer.

This can lead to confusion as users might expect the function to register the address given by the operator parameter and not the caller of the transaction.