add well-known config for OIDC (#15355)

* add well-known config for OIDC

* spacing per feedback

* Update oidc_wellknown.tmpl

* add id_token

* Update oidc_wellknown.tmpl

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

.editorconfig

 [templates/swagger/v1_json.tmpl]
 indent_style = space
 
+[templates/user/auth/oidc_wellknown.tmpl]
+indent_style = space
+
 [Makefile]
 indent_style = tab
 

routers/routes/web.go

 	// Routers.
 	// for health check
 	m.Get("/", routers.Home)
+	m.Get("/.well-known/openid-configuration", user.OIDCWellKnown)
 	m.Group("/explore", func() {
 		m.Get("", func(ctx *context.Context) {
 			ctx.Redirect(setting.AppSubURL + "/explore/repos")

routers/user/oauth.go

 	ctx.Redirect(redirect.String(), 302)
 }
 
+// OIDCWellKnown generates JSON so OIDC clients know Gitea's capabilities
+func OIDCWellKnown(ctx *context.Context) {
+	t := ctx.Render.TemplateLookup("user/auth/oidc_wellknown")
+	ctx.Resp.Header().Set("Content-Type", "application/json")
+	if err := t.Execute(ctx.Resp, ctx.Data); err != nil {
+		log.Error("%v", err)
+		ctx.Error(http.StatusInternalServerError)
+	}
+}
+
 // AccessTokenOAuth manages all access token requests by the client
 func AccessTokenOAuth(ctx *context.Context) {
 	form := *web.GetForm(ctx).(*forms.AccessTokenForm)

templates/user/auth/oidc_wellknown.tmpl

+{
+    "issuer": "{{AppUrl | JSEscape | Safe}}",
+    "authorization_endpoint": "{{AppUrl | JSEscape | Safe}}login/oauth/authorize",
+    "token_endpoint": "{{AppUrl | JSEscape | Safe}}login/oauth/access_token",
+    "response_types_supported": [
+        "code",
+        "id_token"
+    ]
+}