Automatically create single-use auth keys

[lucacome]

This pull request removes the need to specify an auth key. The create command will generate a single-use auth key for the exit node.

I think this is more robust and safe than automatically creating an auth key and storing it in the config file, but let me know.

Closes #37

Configuration and Command Updates:

• Removed the AuthKey field from the TailscaleConfig struct in tailout/config/config.go.
• Removed the tailscale-auth-key flag from the buildCreateCommand function in cmd/create.go.

Core Logic Changes:

• Updated the Create method in tailout/create.go to remove the dependency on AuthKey and instead dynamically create a Tailscale API key with specific capabilities.
• Modified the Tailscale up command to use the newly created API key instead of the removed AuthKey.

Documentation:

• Updated the README.md to reflect the removal of AuthKey and the new method for setting up the Tailscale API key.