ipn/ipnlocal: move Ping method from IPN bus to LocalBackend (HTTP)

Change-Id: I61759f1dae8d9d446353db54c8b1e13bfffb3287
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f