Add a default value field for schema.feature

[schmidt-jake]

We use TFMD schemas to construct feature_specs to parse TFRecords. Some records may have missing entries for certain features, and in such cases we may want to allow a default value. This functionality is already supported in tf.io.FixedLenFeature. It would be nice to add some fields to the Feature message to mirror this:

message Feature {
  ...
  oneof default_value_type {
    int64 int_default = ...;
    ...
  }
}