Fix configuration merging for implicit tproxy upstreams.

Change the merging logic so that the wildcard upstream has correct proxy-defaults
and service-defaults values combined into it. It did not previously merge all fields,
and the wildcard upstream did not exist unless service-defaults existed (it ignored
proxy-defaults, essentially).

Change the way we fetch upstream configuration in the xDS layer so that it falls back
to the wildcard when no matching upstream is found. This is what allows implicit peer
upstreams to have the correct "merged" config.

Change proxycfg to always watch local mesh gateway endpoints whenever a peer upstream
is found. This simplifies the logic so that we do not have to inspect the "merged"
configuration on peer upstreams to extract the mesh gateway mode.