feat: provide logout url for openid providers (#1340)

Co-authored-by: kolaente <k@knt.li>
Reviewed-on: https://kolaente.dev/vikunja/api/pulls/1340
Co-authored-by: viehlieb <pf@pragma-shift.net>
Co-committed-by: viehlieb <pf@pragma-shift.net>

pkg/modules/auth/openid/openid.go

@@ -51,6 +51,7 @@ type Provider struct {
 	Key             string `json:"key"`
 	OriginalAuthURL string `json:"-"`
 	AuthURL         string `json:"auth_url"`
+	LogoutURL       string `json:"logout_url"`
 	ClientID        string `json:"client_id"`
 	ClientSecret    string `json:"-"`
 	openIDProvider  *oidc.Provider