tailcfg: add JSON unmarshal helper for view of node/peer capabilities

Many places that need to work with node/peer capabilities end up with a
something-View and need to either reimplement the helper code or make an
expensive copy. We have the machinery to easily handle this now.

Updates #cleanup

Change-Id: Ic3f55be329f0fc6c178de26b34359d0e8c6ca5fc
Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>
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