Add support for `assetsByAssetFolder` Admin API

[const-cloudinary]

Brief Summary of Changes

This PR adds support for the new Admin API function assetsByAssetFolder.

In addition it adds support for the following parameters:

• asset_folder
• unique_display_name
• use_asset_folder_as_public_id_prefix

to the relevant admin and upload API functions.

Please note that the integration test for the new function was set with conditional flag, since by default this API is not available for all accounts.

What does this PR address?

• [ ] GitHub issue (Add reference - #XX)
• [ ] Refactoring
• [x] New feature
• [ ] Bug fix
• [ ] Adds more tests

Are tests included?

• [x] Yes
• [ ] No

Reviewer, please note:

Checklist:

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I ran the full test suite before pushing the changes and all of the tests pass.