GC process for MLMD

[rcrowe-google]

@1025KB @BrianSong
Currently the OSS version of TFX/MLMD has no garbage collection (GC), meaning that it's left up to individual developers or organizations to create their own. Since GC is something that many teams will want, this project aims to develop a standardized process for defining a GC policy and running GC. This could easily just be a cron script.

See:
google/ml-metadata#162 (comment)
tensorflow/tfx#5093
https://github.com/tensorflow/tfx/blob/4a6421b6b99f6529ef2c95437ce3f3f08a5c04d8/tfx/orchestration/metadata.py#L197