Implement a `deposits list` sub-command to list all the imported keystores

[remyroy]

Is your feature request related to a problem? Please describe. As a tool developer on top of various clients and other tools, it would be nice to be able to list all the imported keystore files using a nimbus sub-command similar to how other consensus clients offer this feature.

Describe the solution you'd like Add a deposits list sub-command that would display the list of all the usable imported keystores. A basic implementation could simply return the public key for each imported keystore on a newline.

Describe alternatives you've considered We could do some script to crawl the data-dir, check in there and try to duplicate Nimbus logic in trying to find what is a usable and correctly imported keystore, but ultimately we want Nimbus to tell us what are the keystores that are usable and working for itself. We are thinking about using the keymanager APIs when it will be ready across the board.