Mesh gateways can use hostnames in their tagged addresses (#7999). This is useful
if you were to expose a mesh gateway using a cloud networking load balancer appliance
that gives you a DNS name but no reliable static IPs.
Envoy cannot accept hostnames via EDS and those must be configured using CDS.
There was already logic when configuring gateways in other locations in the code, but
given the illusions in play for peering the downstream of a peered service wasn't aware
that it should be doing that.
Also:
- ensuring that we always try to use wan-like addresses to cross peer boundaries.
For mTLS to work between two proxies in peered clusters with different root CAs,
proxies need to configure their outbound listener to use different root certificates
for validation.
Up until peering was introduced proxies would only ever use one set of root certificates
to validate all mesh traffic, both inbound and outbound. Now an upstream proxy
may have a leaf certificate signed by a CA that's different from the dialing proxy's.
This PR makes changes to proxycfg and xds so that the upstream TLS validation
uses different root certificates depending on which cluster is being dialed.
R.B. Boyer