[Feature] Don't require login or PostHog telemetry in CLI

[johnpyp]

Problem description

For local usage that doesn't interact with Fern's services, auth0 and PostHog aren't actually necessary at all. Just by replacing all the usage of organizations and users with stubs, the clis and everything still work fine to generate sdks.

Why would it be useful?

The current required authentication is burdensome on automation or just using it without having everything broadcast to tracking servers. It's not offline friendly either, when there's no reason it couldn't be.

Describe the solution (optional)

Make authentication through the CLI optional, and offer an easy way to disable telemetry.

Additional context

Add any other context or screenshots.