Make the initial black background color customizable in youtube_player_flutter

[behzodfaiziev]

Problem

The initial black background that appears when loading the YouTube player is currently hardcoded and cannot be customized. This is particularly noticeable during loading or when the video is not playing, making the screen appear completely black.

Proposed Solution

Allow developers to customize the background color of the player by injecting a Color parameter. By default, this can still be black to maintain backward compatibility.

Benefits

• Improves customization for developers.
• Allows for seamless integration with different app themes.
• Enhances user experience, especially in apps with non-black themes.

Steps to Reproduce

• Use youtube_player_flutter in an app with a light theme.
• Observe that the player background remains black during loading.

Suggested API Changes

• Add a backgroundColor property in the player configuration.
• Default value should remain Colors.black.