feat(toggle): add custom checkedIcon and uncheckedIcon support

[AbdullahAhmadAAK]

Issue number: resolves #30996

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What is the current behavior?

ion-toggle uses fixed built-in on/off icons when enableOnOffLabels is enabled, and there is no way to override those icons globally or per component instance.

What is the new behavior?

This PR adds customizable toggle icons with a clear fallback order:

• Adds per-component props: checkedIcon and uncheckedIcon on [ion-toggle].
• Adds global config options: toggleCheckedIcon and toggleUncheckedIcon
• Uses precedence: component props override global config, and global config overrides existing mode defaults.
• Keeps existing defaults unchanged when no custom icons are provided. Proper stylings done for the icons to adjust in toggle.
• Adds unit tests covering prop-over-config precedence and default fallback behavior for both iOS and MD

Does this introduce a breaking change?

• No

This is a non-breaking feature enhancement focused on toggle icon customization and test coverage only.

Screenshot of how it looks

Video : https://github.com/user-attachments/assets/a8a103dc-d6b1-4b81-be65-baa5b476a379

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