Add generic OpenID Connect provider (OAuth2)

This adds the oauth2 provider `oidc`. It needs an additional argument, the OIDC discovery endpoint to figure out where the auth and token URLs are.

Configuration is similar to setting up the Google Authentication with these changes:

 * `OAUTH2_PROVIDER = oidc`
 * `OAUTH2_OIDC_DISCOVERY_ENDPOINT = https://auth.exampe.org/discovery`

ui/oauth2.go

@@ -5,14 +5,17 @@
 package ui // import "miniflux.app/ui"
 
 import (
+	"context"
 	"miniflux.app/config"
 	"miniflux.app/oauth2"
 )
 
-func getOAuth2Manager() *oauth2.Manager {
+func getOAuth2Manager(ctx context.Context) *oauth2.Manager {
 	return oauth2.NewManager(
+		ctx,
 		config.Opts.OAuth2ClientID(),
 		config.Opts.OAuth2ClientSecret(),
 		config.Opts.OAuth2RedirectURL(),
+		config.Opts.OAuth2OidcDiscoveryEndpoint(),
 	)
 }

ui/oauth2_callback.go

@@ -44,14 +44,14 @@ func (h *handler) oauth2Callback(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	authProvider, err := getOAuth2Manager().Provider(provider)
+	authProvider, err := getOAuth2Manager(r.Context()).Provider(provider)
 	if err != nil {
 		logger.Error("[OAuth2] %v", err)
 		html.Redirect(w, r, route.Path(h.router, "login"))
 		return
 	}
 
-	profile, err := authProvider.GetProfile(code)
+	profile, err := authProvider.GetProfile(r.Context(), code)
 	if err != nil {
 		logger.Error("[OAuth2] %v", err)
 		html.Redirect(w, r, route.Path(h.router, "login"))

ui/oauth2_redirect.go

@@ -24,7 +24,7 @@ func (h *handler) oauth2Redirect(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	authProvider, err := getOAuth2Manager().Provider(provider)
+	authProvider, err := getOAuth2Manager(r.Context()).Provider(provider)
 	if err != nil {
 		logger.Error("[OAuth2] %v", err)
 		html.Redirect(w, r, route.Path(h.router, "login"))

ui/oauth2_unlink.go

@@ -24,7 +24,7 @@ func (h *handler) oauth2Unlink(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	authProvider, err := getOAuth2Manager().Provider(provider)
+	authProvider, err := getOAuth2Manager(r.Context()).Provider(provider)
 	if err != nil {
 		logger.Error("[OAuth2] %v", err)
 		html.Redirect(w, r, route.Path(h.router, "settings"))