ipn/localapi: add debug-web-client endpoint

Debug endpoint for the web client's auth flow to talk back to the
control server. Restricted behind a feature flag on control.

We will either be removing this debug endpoint, or renaming it
before launching the web client updates.

Updates tailscale/corp#14335

Signed-off-by: Sonia Appasamy <sonia@tailscale.com>
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