Add read_avro and list_avro_columns for rework on Splittable Avro support

This PR is part of the effort to rework on Dataset with large files reading into Tensors first to speed up performance. See 382 and 366 for related discussions.

Summary:

1) read_avro is able to read a avro file within the range of [offset, offset+length] (Splittable)
2) we use primitive read_avro C++ ops to read in big chunks and then wire up with tf.data.Dataset
3) read_avro could be used in other places.
4) AvroDataset automatically find out the dtype in eager mode, in graph mode, user has
   to specify the dtype in kwargs.

Signed-off-by: Yong Tang <[email protected]>

Author: yongtang

tensorflow_io/avro/BUILD

@@ -10,7 +10,7 @@ load(
 cc_library(
     name = "avro_ops",
     srcs = [
-        "kernels/avro_input.cc",
+        "kernels/avro_kernels.cc",
         "ops/avro_ops.cc",
     ],
     copts = tf_io_copts(),

tensorflow_io/avro/__init__.py

@@ -15,18 +15,24 @@
 """Avro Dataset.
 
 @@AvroDataset
+@@list_avro_columns
+@@read_avro
 """
 
 from __future__ import absolute_import
 from __future__ import division
 from __future__ import print_function
 
 from tensorflow_io.avro.python.ops.avro_ops import AvroDataset
+from tensorflow_io.avro.python.ops.avro_ops import list_avro_columns
+from tensorflow_io.avro.python.ops.avro_ops import read_avro
 
 from tensorflow.python.util.all_util import remove_undocumented
 
 _allowed_symbols = [
     "AvroDataset",
+    "list_avro_columns",
+    "read_avro_",
 ]
 
 remove_undocumented(__name__, allowed_exception_list=_allowed_symbols)