If a user explicitly adds a non-ts.net (not a CertDomain domain) domain
like "foo.com" to their serve config as a web target that's also an allowed
funnel domain (using raw "tailscale serve set-config"), then use the new
ALPN cert fetching (from b553969b) to get certs for that domain.
This is just plumbing; there's no new product functionality to
actually enable this easily client-side, and it also has no visible
product surface to enable it server-side.
Updates tailscale/corp#41736
Change-Id: Ie2e421ac9611bce64bba3de6a454b2d505ea0e8a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
412c812d76