Add datepicker widget

[hazemhashem]

Description:

This PR introduces a new DatePicker widget to allow users to select dates using dropdowns for year, month, and day. The widget is fully customizable and integrates seamlessly with the existing Fyne API.
Fixes #(issue)

Checklist:

• Tests included.
  • I have added unit tests for the DatePicker widget in datepicker_test.go.
• Lint and formatter run with no errors.
  • I ran go fmt and golint with no errors.
• Tests all pass.
  • All tests pass locally. Here's the output of go test -v -run TestDatePicker:

=== RUN TestDatePicker
--- PASS: TestDatePicker (0.00s)
PASS
ok fyne.io/fyne/v2/widget 0.047s

```

Output :

[andydotxyz]

How does this fit alongside the DateEntry?

I'm not sure about the 3 separate pickers - on mobile aren't these presented in a single keyboard/picker instead of directly in the user interface?

https://assets.website-files.com/6231812313e8661d7da3a4e7/62820ad35553f03e5fb5e1a0_13-Good-user-experience-datepicker-examples.png

[hazemhashem]

You're absolutely right about the mobile usability concern. It slipped my mind because I don’t currently have an Android/iOS development environment to test mobile interfaces. My initial implementation is focused on desktop use cases, such as sign-up forms or other scenarios where a structured date selection might be more intuitive.

For desktop applications, having 3 separate pickers (year, month, day) can be beneficial in scenarios like:

• Sign-up forms: Users can quickly select their date of birth without needing to open a popup.
• Filters or dashboards: Users can easily adjust date ranges without navigating through a calendar.

I’d love to hear your thoughts on the following:

• Should the 3 separate pickers remain as a single widget for desktop use?
• Would it be better to move them into a custom dialog for better organization?
• Should we consider adapting this for mobile use in a future update?

Your guidance would be greatly appreciated as I want to ensure this widget aligns with Fyne’s design principles and usability standards.