feat: datetime, allow for optional year selection

### Prerequisites

- [X] I have read the [Contributing Guidelines](https://github.com/ionic-team/ionic-framework/blob/main/.github/CONTRIBUTING.md#creating-an-issue).
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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

The new ion-datetime component should support selection of a date without a specific year. If I add a birthday or a date (like named where year selection makes no sense) on the native address book of iOS (and a few android flavours I tried) the picker has option for year "----". The selected date then shows as Day-Month only with no year element.



### Describe the Use Case

Select a date that is not defined by a year, such as a recurring date or range that does not have a defined start year or the year is not important.

### Describe Preferred Solution

An attribute that indicates the year selection is optional would be the ideal solution I believe. The Datetime component then would have an extra year option denoted as '----' which could set the year to an extreme (say 9999) to differentiate from an explicit year selection since a valid date has to be returned by the component in all modes.

### Describe Alternatives

_No response_

### Related Code

_No response_

### Additional Information

_No response_

Start by reading the ion-datetime component and its current date-value behavior. Define how an optional year is represented and how the picker displays the year-less option, then verify that recurring dates can be selected and returned as valid dates without an explicit year.

Written by the indexing model from the issue text.