feat: ion-toggle: Custom on/off icons

### Prerequisites

- [x] I have read the [Contributing Guidelines](https://github.com/ionic-team/ionic-framework/blob/main/docs/CONTRIBUTING.md#creating-an-issue).
- [x] I agree to follow the [Code of Conduct](https://ionicframework.com/code-of-conduct).
- [x] I have searched for [existing issues](https://github.com/ionic-team/ionic-framework/issues) that already include this feature request, without success.

### Describe the Feature Request

Currently it is impossible to provide our own icons on the toggles, and this leads to problems with styled toggles as they can seem uncanny (eg. a M3 Expressive toggle using M2 icons).

I would love for the toggle to have a way for me to provide its icons.

### Describe the Use Case

Toggle customization, but also some may use other kinds of icons also.

### Describe Preferred Solution

_No response_

### Describe Alternatives

_No response_

### Related Code

_No response_

### Additional Information

_No response_

Start by locating the ion-toggle component and its existing rendering and test entry points; the issue names no files or tests. Determine how custom on/off icons should be exposed and verify that both toggle states retain their expected behavior without breaking the default icons.

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