Use the parsed and validated advertise tags value from prefs instead of
doing a strings.Split on the raw tags value as an input to the OAuth and
identity federation auth key generation methods.
The previous strings.Split method would return an array with a single
empty string element which would pass downstream length checks on the
tags argument before eventually failing with a confusing message when
hitting the API.
Fixes https://github.com/tailscale/tailscale/issues/18617
Signed-off-by: Mario Minardi <mario@tailscale.com>
6587cafb3f