Provide fallback method if local server is not reachable

It would be nice if oauth2cli provides a fallback method if the local server is not reachable such as a remote host. This issue is originally from https://github.com/int128/kubelogin/issues/148.

User interaction

Open http://localhost:8000
If you cannot open the browser, visit https://accounts.google.com/... and enter code: xxxxxxxx

Authorization will finish when the local server received a code or user entered a code by keyboard.

Implementation consideration

Redirect URIs are different between the local server and keyboard as:

• By the local server: redirect_uri=http://localhost:8000
• By keyboard: redirect_uri=urn:ietf:wg:oauth:2.0:oob

Note that post-process (exchanging code) must use the corresponding redirect URI, or error will be returned.

It may be complex design as:

1. Goroutine 1 serves the local server.
2. Goroutine 2 waits for an authorization response and shuts down the local server.
3. Goroutine 3 waits for a code from keyboard and shuts down the local server.

Unfortunately fmt.Scanf() and io.Stdin.Read() do not support a context and there is no way to break an interaction for now (as https://github.com/golang/go/issues/20280). So the goroutine3 will remain after the authorization.

See also: https://github.com/int128/kubelogin/blob/1.0/oidc.go#L81

Start by reading the OAuth flow referenced in kubelogin's oidc.go at line 81 and the issue's redirect URI requirements. Define how the local-server and keyboard paths coordinate, including server shutdown and code exchange. Done means authorization can complete through either the local server or the remote keyboard fallback with the matching redirect URI.