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.
I noticed that the JSON api endpoints for peerings json encodes protobufs directly, rather than converting them into their `api` package equivalents before marshal/unmarshaling them.
I updated this and used `mog` to do the annoying part in the middle.
Other changes:
- the status enum was converted into the friendlier string form of the enum for readability with tools like `curl`
- some of the `api` library functions were slightly modified to match other similar endpoints in UX (cc: @ndhanushkodi )
- peeringRead returns `nil` if not found
- partitions are NOT inferred from the agent's partition (matching 1.11-style logic)
Freddy