config get: support getting oauth_token from keyring

Since we've advertised `gh config get -h HOST oauth_token` as an API for outside scripts and extensions to access the token, this should continue working even if the token was technically stored in keyring and not in the config file.

pkg/cmd/config/get/get.go

@@ -1,6 +1,7 @@
 package get
 
 import (
+	"errors"
 	"fmt"
 
 	"github.com/MakeNowJust/heredoc"
@@ -53,6 +54,16 @@ func NewCmdConfigGet(f *cmdutil.Factory, runF func(*GetOptions) error) *cobra.Co
 }
 
 func getRun(opts *GetOptions) error {
+	// search keyring storage when fetching the `oauth_token` value
+	if opts.Hostname != "" && opts.Key == "oauth_token" {
+		token, _ := opts.Config.Authentication().Token(opts.Hostname)
+		if token == "" {
+			return errors.New(`could not find key "oauth_token"`)
+		}
+		fmt.Fprintf(opts.IO.Out, "%s\n", token)
+		return nil
+	}
+
 	val, err := opts.Config.GetOrDefault(opts.Hostname, opts.Key)
 	if err != nil {
 		return err