Add Schema support for Tensorflow

[damccorm]

Similar to BEAM-5807, Tensorflow's defacto storage format is TFRecord files with Example proto payload and its own schema.proto. We already have TFRecordIO support. Need to implement:

• Conversion between Beam and TF schema
• Conversion between Beam Row and TF Example proto
• TFRecordTableProvider

https://github.com/tensorflow/metadata/blob/master/tensorflow_metadata/proto/v0/schema.proto

https://github.com/tensorflow/tensorflow/blob/master/tensorflow/core/example/example.proto

 

Also it seems the metadata protos are not published as Java artifacts:

tensorflow/metadata#5

My WIP branch: https://github.com/spotify/beam/tree/neville/tf

Imported from Jira BEAM-7921. Original Jira may contain additional context.
Reported by: sinisa_lyh.