Fix rest api task extractor, add job export function

[zhiltsov-max]

Motivation and context

• Fixed REST API extractor for Datumaro format export
• Added job annotations export function

How has this been tested?

Manual testing

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG file
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (read github docs)
• I have increased versions of npm packages if it is necessary (cvat-canvas,
  cvat-core, cvat-data and cvat-ui)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2020 Intel Corporation
#
# SPDX-License-Identifier: MIT

[coveralls]

Pull Request Test Coverage Report for Build 6790

• 3 of 11 (27.27%) changed or added relevant lines in 1 file are covered.
• 152 unchanged lines in 6 files lost coverage.
• Overall coverage increased (+0.005%) to 69.066%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
cvat/apps/dataset_manager/task.py	3	11	27.27%

Files with Coverage Reduction	New Missed Lines	%
cvat/apps/engine/urls.py	5	76.67%
cvat/apps/engine/media_extractors.py	6	75.81%
cvat/apps/authentication/views.py	8	64.29%
datumaro/datumaro/plugins/coco_format/converter.py	19	89.76%
datumaro/datumaro/components/extractor.py	33	90.92%
cvat/apps/engine/views.py	81	78.64%

Totals
Change from base Build 6698:	0.005%
Covered Lines:	11143
Relevant Lines:	15631

---

💛 - Coveralls

[nmanovic]

@azhavoro , could you please look at the PR and test it?

[azhavoro]

@zhiltsov-max I'm faced with the following error:

datum project export -o /tmp/t1 -f voc -- --save-images
2020-08-04 17:03:37,496 INFO: Loading the project...
2020-08-04 17:03:37,543 INFO: Exporting the project...
2020-08-04 17:03:37,544 ERROR: 'cvat_rest_api_task_images' object has no attribute '_session'
Traceback (most recent call last):
  File "/home/andrey/Downloads/datum/.venv/bin/datum", line 11, in <module>
    load_entry_point('datumaro', 'console_scripts', 'datum')()
  File "/home/andrey/workspace/intel/cvat_yg_disk/datumaro/datumaro/cli/__main__.py", line 114, in main
    return args.command(args)
  File "/home/andrey/workspace/intel/cvat_yg_disk/datumaro/datumaro/cli/contexts/project/__init__.py", line 357, in export_command
    **filter_args)
  File "/home/andrey/workspace/intel/cvat_yg_disk/datumaro/datumaro/components/project.py", line 741, in export_project
    converter(dataset, save_dir)
  File "/home/andrey/workspace/intel/cvat_yg_disk/datumaro/datumaro/cli/contexts/project/__init__.py", line 345, in converter_proxy
    return converter.convert(extractor, save_dir, **extra_args)
  File "/home/andrey/workspace/intel/cvat_yg_disk/datumaro/datumaro/components/converter.py", line 37, in convert
    return converter.apply()
  File "/home/andrey/workspace/intel/cvat_yg_disk/datumaro/datumaro/plugins/voc_format/converter.py", line 109, in apply
    self.save_subsets()
  File "/home/andrey/workspace/intel/cvat_yg_disk/datumaro/datumaro/plugins/voc_format/converter.py", line 172, in save_subsets
    osp.join(self._images_dir, image_filename))
  File "/home/andrey/workspace/intel/cvat_yg_disk/datumaro/datumaro/components/converter.py", line 65, in _save_image
    image = item.image.data
  File "/home/andrey/workspace/intel/cvat_yg_disk/datumaro/datumaro/util/image.py", line 221, in data
    return self._data()
  File "/home/andrey/workspace/intel/cvat_yg_disk/datumaro/datumaro/util/image.py", line 178, in __call__
    image = self.loader(self.path)
  File "/home/andrey/Downloads/datum/.datumaro/plugins/cvat_rest_api_task_images.py", line 33, in <lambda>
    lambda item_id: self._image_loader(item_id, self))
  File "/home/andrey/Downloads/datum/.datumaro/plugins/cvat_rest_api_task_images.py", line 62, in _image_loader
    extractor._download_image(item_id)
  File "/home/andrey/Downloads/datum/.datumaro/plugins/cvat_rest_api_task_images.py", line 39, in _download_image
    self._connect()
  File "/home/andrey/Downloads/datum/.datumaro/plugins/cvat_rest_api_task_images.py", line 45, in _connect
    if self._session is not None:
AttributeError: 'cvat_rest_api_task_images' object has no attribute '_session'