Create github token from tkn-pac

[chmouel]

We currently send folks to a web browser to create token,

We can definitively do it the same way gh auth does it :

it redirect to this page :

https://github.com/login/device

which then it use its internal library authflow :

https://github.com/cli/cli/blob/trunk/internal/authflow/flow.go

to validate it and get the token

we would probably need to create our own here:

https://github.com/cli/cli/blob/trunk/internal/authflow/flow.go#L21-L24

and we can reuse the same logic for Github Enteprise...