fix: Restore accounts:add and accounts:set functionality

[zwhitfield3]

Description

Bug: W-18997338
Issue: #3328

Since the migration of the Heroku Accounts plugin, the accounts:add command stopped prompting for a new login. This meant every time the command was executed, it added the same account credentials to a different account name, preventing new accounts from being added or set with accounts:set. This PR fixes the issue by re-introducing the CLI login flow required to add new accounts. This PR also reinstates the --sso functionality the plugin provided.

Testing

Note: You'll need access to 2 Heroku accounts. The SSO flow already works with federated accounts and part of this PR just reinstates the functionality. We need to test that new accounts can be added and set.

1. Pull down branch & run yarn && yarn build
2. Run ./bin/run accounts:add personal - confirm the login flow is prompted with successful login into first Heroku account
3. Logout of Heroku account
4. Run ./bin/run accounts:add personal2 - confirm the login flow is prompted with successful login into second Heroku account
5. Run ./bin/run accounts - confirm both accounts exist and personal2 is currently selected
6. Run ./bin/run accounts:set personal
7. Run ./bin/run accounts:current - confirm personal is the currently selected account