cvat-sdk: Fix creating tasks with non-local files

[SpecLad]

Forcing the Content-Type for the upload_data API call to multipart/form-data does not work, because the current logic for
converting Python values to parts (ApiClient._convert_body_to_post_params) does not encode them in a way that Django REST Framework can understand (it JSON-encodes each part).

Fortunately, we don't actually need to do that, since when we create a task with non-local files, we don't need to upload any files, and so we can just post the original JSON, so do just that.

I couldn't add a test for the remote image case, because CVAT rejects all URLs with non-public IP addressses. However, I did test this case manually.

Motivation and context

Fixes #4962.

How has this been tested?

SDK unit tests + manual testing for remote files.

Checklist

• I submit my changes into the develop branch
• I have added a 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.

[zhiltsov-max]

The implementation looks OK to me. However, I'd like to ask for few other changes:

1. The tests/share is better be called tests/shared_drive or similar. Otherwise, it looks like it's just a directory with shared data for tests (which is also needed, in perspective)
2. Probably, we need to add a similar test in the rest_api tests layer, there is no such test yet.
3. Probably, we should check task creation with more params. Maybe just add more/all params in the existing test, or add a new one.

[SpecLad]

1. The tests/share is better be called tests/shared_drive or similar. Otherwise, it looks like it's just a directory with shared data for tests (which is also needed, in perspective)

Sure, I can do that.

2. Probably, we need to add a similar test in the rest_api tests layer, there is no such test yet.
3. Probably, we should check task creation with more params. Maybe just add more/all params in the existing test, or add a new one.

I think that goes beyond the scope of this PR. The tests I added are sufficient to make sure that the bugfix works; adding other tests would be a separate task.

[azhavoro]

This PR broke e2e test case_107_connected_file_share

CypressError: `cy.exec('mv cypress/integration/actions_tasks3/assets/case_107/image_case_107_1.png cypress/integration/actions_tasks3/assets/case_107/image_case_107_1.png.bk')` failed because the command exited with a non-zero code.

Information about the failure:
Code: 1

Stderr:
mv: cannot stat 'cypress/integration/actions_tasks3/assets/case_107/image_case_107_1.png': No such file or directory

[SpecLad]

My bad; didn't see that usage. I'll try to fix it.