Protocol buffer definitions for xDS and top-level resource API messages.

Package group //envoy/api/v2:friends enumerates all consumers of the shared API messages. That includes package envoy.api.v2 itself, which contains several xDS definitions. Default visibility for all shared definitions should be set to //envoy/api/v2:friends.

Additionally, packages envoy.api.v2.core and envoy.api.v2.auth are also consumed throughout the subpackages of //envoy/api/v2.