tsnet: add Server.AuthKey field

... so callers can provide the AuthKey via mechanisms other than
environment variables which means multiple Servers can't be started
concurrently in the same process without coordination.

Change-Id: I7736ef4f59b7cc29637939e140e990613ce58e0d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f