feat: add method to auth with a token directly

[mortenpi]

Adds a second method to JuliaHub.authenticate that can be used to authenticate JuliaHub.jl directly with a token (rather than interactive auth or reading the auth.toml). Useful for e.g. CI, where you might have the token in an env. variable.

julia> JuliaHub.authenticate("juliahub.com", "eyJhbGciO...IJw")
JuliaHub.Authentication("https://juliahub.com", "mortenpi", *****)

cc @pmodv