Implement hlx auth --github

[tripodsan]

Description
there should be a simple way to retrieve the credentials that are required for depoyment from the various backends (see #380 (comment))

this issue is about adding hlx auth --github which will initiate the authentication process with github for the user to generate a token as helix bot. see https://github.com/adobe/helix-bot/issues/33 for details.

[tripodsan]

example output:

$ hlx auth --github

Thank you for choosing the automatic Helix Bot Authentication process.
I will shortly open a browser with the following url. In case the browser doesn't
open automatically, please navigate to the url manually:

   https://app.project-helix.io/login/start?p=1234.

Oh, and I started local server, bound to 127.0.0.1:1234 and am awaiting the completion 
of the process.

<<spinner spinning>>

The authentication process has completed. Thank you. 
I received an access token that I can use to access the Helix Bot on your behalf. 

If you wish, I can add it automatically to a file [Y/n]: ?
which file: [.env]: ?

ok. I added it to `.env`.

----< or >-----
ok. here's the token for you:
HLX_GITHUB_TOKEN=fsdfgsdfgsdfgdsfgdsfg

You can now close the browser window if it didn't close automatically.

[tripodsan]

would need #580

[trieloff]

Sounds good. When you ask the "should I save this stuff" question, check first that this is an interactive (TTY) environment.

[tripodsan]

...check first that this is an interactive (TTY) environment.

I think that the entire interactive hlx auth should only work on a TTY.