tsnet,internal/client/tailscale: resolve OAuth into authkeys in tsnet (#17191)

* tsnet,internal/client/tailscale: resolve OAuth into authkeys in tsnet

Updates #8403.

* internal/client/tailscale: omit OAuth library via build tag

Updates #12614.

Signed-off-by: Naman Sood <mail@nsood.in>

feature/condregister/oauthkey/doc.go

@@ -0,0 +1,10 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package oauthkey registers support for OAuth key resolution
+// if it's not disabled via the ts_omit_oauthkey build tag.
+// Currently (2025-09-19), tailscaled does not need OAuth key
+// resolution, only the CLI and tsnet do, so this package is
+// pulled out separately to avoid linking OAuth packages into
+// tailscaled.
+package oauthkey

feature/condregister/oauthkey/maybe_oauthkey.go

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