spinner: Add support for custom easing function

[dunkmann00]

Description

Adds a way to customize the easing function for a spinner.

I also added some examples to the spinner story.

Checklist

• I have read the CONTRIBUTING document and followed the guidelines.
• Reviewed the changes in this PR and confirmed AI generated code (If any) is accurate.
• Passed cargo run for story tests related to the changes.
• Tested macOS, Windows and Linux platforms performance (if the change is platform-specific)

[madcodelife]

Thank you!