Feature: Allow authentication via API Credentials

Currently, we only support authentication via a user token (which is based on a cookie). We are in the process of opening up the Accounts APIs (cc @utsavoza) where we would like any user to access the endpoints present alternatively without using a token.

We can add this support in the CLI itself by offering:

```
abc login api
```

which will login the user using their master API credentials, cc @utsavoza.