cmd/tailscale/cli,feature: add support for identity federation (#17529)

Add new arguments to `tailscale up` so authkeys can be generated dynamically via identity federation.

Updates #9192

Signed-off-by: mcoulombe <max@tailscale.com>

feature/condregister/identityfederation/doc.go

@@ -0,0 +1,7 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package identityfederation registers support for authkey resolution
+// via identity federation if it's not disabled by the
+// ts_omit_identityfederation build tag.
+package identityfederation

feature/condregister/identityfederation/maybe_identityfederation.go

@@ -0,0 +1,8 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !ts_omit_identityfederation
+
+package identityfederation
+
+import _ "tailscale.com/feature/identityfederation"