Nicknames/profiles for managing multiple credentials.

[aepace]

Description

When managing multiple credentials, it would be useful to have a user-defined nickname or profile name for each set of credentials. Especially since there's no way to define a memorable username outside of the, for example, albert.einstein-abcd1234 formatting construct.

Consider:

1. Having the nickname be an optional interactive input when the user runs hop auth add. The nickname = XXX field would then be in the same block as username = ... and password = ... in the auth.toml file.
2. Being able to create a new Auth object by specifying the credential nickname, and then hop-client would parse the auth.toml file for the nickname and then use the corresponding username and password.