Select frame for thumbnail extraction

[Dreamink-Official]

Changes: Instead of using the middle frame of the video, the user can now select a specific frame for thumbnail generation. If no valid value is provided, the middle frame will still be used as a fallback.

Changelog Description

• Added get_attribute_defs() to introduce the frame selection field in the publisher.
• Created _get_thumbnail_frame() to retrieve the correct frame.
• Adjusted _create_frame_from_video() to select the specified frame from the video instead of using timestamps.

Additional info

This PR is related to #1051.
As mentioned in the issue, I am not entirely sure which option aligns best with the default behavior of AYON:
A. Enter the real frame number to be used as the thumbnail (e.g., 1005) (my preferred method).
B. Enter the frame number of the extracted video (e.g., 5).

In the first commit, I used my preferred method [A].

Testing notes:

1. Create a product type that also generates a thumbnail.
2. Enter your frame number.
3. Publish.