appc,cmd/sniproxy,ipn/ipnlocal: split sniproxy configuration code out of appc

The design changed during integration and testing, resulting in the
earlier implementation growing in the appc package to be intended now
only for the sniproxy implementation. That code is moved to it's final
location, and the current App Connector code is now renamed.

Updates tailscale/corp#15437

Signed-off-by: James Tucker <james@tailscale.com>
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