Add Update Withdrawal Credentials option

[alex-miller-0]

It appears the ETH2 spec has moved on from BLS withdrawal keys and will force a transition from BLS -> ETH1 withdrawal credentials for 0x00 type credentials in order to do any withdrawals. I suppose this makes sense because you need to be able to withdraw to an execution-layer address.

In the upcoming Capella fork, there is a new method to allow updating of withdrawal credentials.

The method is called SignedBLSToExecutionChange.

The workflow should look like:

1. Ask for derivation path -> find deposit key and withdrawal key
2. Obtain validator index associated with deposit key. This can either be done by a lookup from the CLI using e.g. Beaconscan OR by asking the user to input the index. We could also attempt to lookup on Beaconscan and fall back to the validator index.
3. Ask for the new ETH1 address to be added to the credentials
4. Form message and request signature

NOTE: We could also add another screen to the Lattice for updating credentials, just like for generating deposit data