This PR adds `login` and `logout` commands to the `pulumi` CLI.
Rather than requiring a user name and password like before, we instead require users to login with GitHub credentials on the Pulumi Console website. (You can do this now via https://beta.moolumi.io.) Once there, the account page will show you an "access token" you can use to authenticate against the CLI.
Upon successful login, the user's credentials will be stored in `~/.pulumi/credentials.json`. This credentials file will be automatically read with the credentials added to every call to `PulumiRESTCall`.
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