cmd/cue: add a cue login --token flag to log into a registry with a token generated via the web UI

[mvdan]

Thanks to the work that @rustamabd2 has done on the server side over the last week, we can now generate tokens via the web UI. We need a command like cue login --token to be able to add them to $CUE_CONFIG_DIR/logins.json easily.

With v1 tokens, the token generated by the web UI is simply an access token, so this would result in an entry in $CUE_CONFIG_DIR/logins.json with just access_token but no expiry. We're discussing v2 tokens on the backend side to embed the expiry timestamp as part of the encoded token to fix this minor issue.