OAuth2Strategy interface to abstract the details of the login flow

This allows implementing clients the deviate from the standard OAuth2 flow

internal/auth/strategy.go

@@ -0,0 +1,56 @@
+/*
+Copyright © 2023 Philipp Wolfer <phw@uploadedlobster.com>
+
+Scotty is free software: you can redistribute it and/or modify it under the
+terms of the GNU General Public License as published by the Free Software
+Foundation, either version 3 of the License, or (at your option) any later version.
+
+Scotty is distributed in the hope that it will be useful, but WITHOUT ANY
+WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+A PARTICULAR PURPOSE. See the GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License along with
+Scotty. If not, see <https://www.gnu.org/licenses/>.
+*/
+
+package auth
+
+import (
+	"context"
+
+	"golang.org/x/oauth2"
+)
+
+type OAuth2Strategy interface {
+	Config() oauth2.Config
+
+	AuthCodeURL(verifier string, state string) string
+
+	ExchangeToken(code CodeResponse, verifier string) (*oauth2.Token, error)
+}
+
+type CodeResponse struct {
+	Code  string
+	State string
+}
+
+func NewStandardStrategy(conf oauth2.Config) OAuth2Strategy {
+	return StandardStrategy{conf: conf}
+}
+
+type StandardStrategy struct {
+	conf oauth2.Config
+}
+
+func (s StandardStrategy) Config() oauth2.Config {
+	return s.conf
+}
+
+func (s StandardStrategy) AuthCodeURL(verifier string, state string) string {
+	return s.conf.AuthCodeURL(state, oauth2.AccessTypeOffline, oauth2.S256ChallengeOption(verifier))
+}
+
+func (s StandardStrategy) ExchangeToken(code CodeResponse, verifier string) (*oauth2.Token, error) {
+	ctx := context.Background()
+	return s.conf.Exchange(ctx, code.Code, oauth2.VerifierOption(verifier))
+}