Also, change the ProxyState.id to identity. This is because we already have the id of this proxy
from the resource, and this id should be name-aligned with the workload it represents. It should
also have the owner ref set to the workload ID if we need that. And so the id field seems unnecessary.
We do, however, need a reference to workload identity so that we can authorize the proxy when it initially
connects to the xDS server.
Configuration that previously was inlined into the Upstreams resource
applies to both explicit and implicit upstreams and so it makes sense to
split it out into its own resource.
It also has other minor changes:
- Renames `proxy.proto` proxy_configuration.proto`
- Changes the type of `Upstream.destination_ref` from `pbresource.ID` to
`pbresource.Reference`
- Adds comments to fields that didn't have them