[datareposink] Add function to write flexible tensors

[datareposink] Add function to write flexible tensors

- To add memory to gstbuffer by number of flexible tensors when read sample in datareposrc,
  sample_offset, tensor_size, and tensor_count field are added to the JSON file
- Add unit test

- Reference
  * If caps of sink pad is flexible, the input gstBuffer is saved as a flexible tensor
  * The size of the input gstBuffer is stored as sample_offset field in JSON file and
    it will be used when shuffle operation in datareposrc
  * Each flexible tensor size is stored as tensor_size field in JSON file and
    it will be used with gst_buffer_append_memory() in datareposrc

Signed-off-by: hyunil park <hyunil46.park@samsung.com>