feat: datetime, customize how "today" is calculated with grid buttons

### Prerequisites

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- [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

If user has custom timezone (not the same as on device) we dont have ability to highlight the current day due to that timezone today

### Describe the Use Case

We need a property "timezone" which will render current date due to that timezone. It could be passed as UTC offset in minutes with negative or positive value. and based on that calendar will render proper current date

### Describe Preferred Solution

_No response_

### Describe Alternatives

_No response_

### Related Code

_No response_

### Additional Information

_No response_

The issue names datetime and grid buttons but provides no files, tests, or entry points. Start by locating the datetime component's current-day calculation and grid rendering, then determine how a timezone offset should affect the highlighted date and verify that behavior with focused tests.

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