tailcfg: add ServiceName

Rather than using a string everywhere and needing to clarify that the
string should have the svc: prefix, create a separate type for Service
names.

Updates tailscale/corp#24607

Change-Id: I720e022f61a7221644bb60955b72cacf42f59960
Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>
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