Utility to create a feature_spec from a schema

[schmidt-jake]

Converting a schema to a feature_spec = {feature.name: tf.io.FixedLenFeature(...) for feature in schema.feature} is a very common task. There is a utility to do this already in tensorfow transform, but there are many times where I don't need tensorflow transform except for that one function. I think it should belong in this repo instead.