CLI: Memberships management

There's a need to manage not only the accounts (keys), but also the available memberships and roles inside the CLI in order to allow the user to specify the membership and role as a context for executing some of the commands that are going be implemented in the future (related to voting and governance).

I already started implementing this with the assumption that each account (pair of keys) always has either one or zero related memberships, so the base memership and available roles can be determined based on the currently selected account, but this may require some more inspection.

Some initial ideas I've had for commands that would need to be included:

• member:register - create a membership profile for the currently selected account (if it doesn't already exist)
• member:current - display information about currently used membership (and available roles)
• member:edit - edit the current membership profile

There are definitely going to be more, but I'm still trying to figure out the correct way to handle this. I will update the issue as I go along.

Some commands for discovering other members will probably also need to exists, like listing, reading other members profiles etc., but I think that can be handled in a separate issue (as this one is focused mostly on how to manage user's own memberships)

Start by reviewing the existing CLI account and key-management entry points, then compare the proposed member:register, member:current, and member:edit commands with the membership and role model. Confirm how the selected account relates to memberships before defining the command scope; done means an agreed design for managing the current membership and its available roles.