Add user whoami command

[bgins]

Summary

Problem

There is currently no way to check that you are signed in and the username you are signed in with within the Rust code base.

Impact

It can be confusing if you have multiple accounts to know which one you are using without this feature. It also helps quickly ensure that the sign in process did work for sign in

Solution

Write a command that sends a request to the server that gets the username that's tied to the DID that was sent.

Details

This feature is to be added during the Cli rewrite in Rust.