Add a recursive message type

A bug in gapic-generator-python recently emerged due to recursive message types: https://github.com/googleapis/gapic-generator-python/issues/585

Hand testing indicates that the issue is solved, but regressions may be possible, and other generators may not .
Constructing recursive message types in unit tests is complicated and unergonomic due to the protobuf descriptor interface. Having a test possible via showcase would ameliorate the issue.