ipn/ipnlocal: start adding DoH DNS server to peerapi when exit node

Updates #1713

Change-Id: I8d9c488f779e7acc811a9bc18166a2726198a429
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