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
Iryna Shustava