feat: ion-alert add a slot for full customization

### 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

I would like to replicate this loading alert that you get when sharing a video from the photos app.

however currently, there are no slots to alert so this is not possible

### Describe the Use Case

one use case is to add icons like in this closed issue https://github.com/ionic-team/ionic-framework/issues/7874

### Describe Preferred Solution

using a default slot is probably the most flexible solution, then the user can have whatever they want inside such as Ion-icon and animated stuff

### Describe Alternatives

_No response_

### Related Code

_No response_

### Additional Information

_No response_

Start by locating the ion-alert component and its existing rendering and test entry points. Review how alert content is currently represented, then add support for a default slot that accepts custom content such as icons or animations; done means the requested loading-alert layout is possible and existing alert behavior remains intact.

索引モデルが issue の本文から書いたものです。