Preserve current behaviour with a config flag

Add a configuration flag (default true to preserve current behaviour) to
allow headscale to start without OIDC being able to initialise.

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

app.go

@@ -192,7 +192,9 @@ func NewHeadscale(cfg *Config) (*Headscale, error) {
 
 	if cfg.OIDC.Issuer != "" {
 		err = app.initOIDC()
-		if err != nil {
+		if err != nil && cfg.OIDC.OnlyStartIfOIDCIsAvailable {
+			return nil, err
+		} else {
 			log.Warn().Err(err).Msg("failed to set up OIDC provider, falling back to CLI based authentication")
 		}
 	}