feat: Expose ion-datetime header styling

[clowcher]

Prerequisites

• I have read the Contributing Guidelines.
• I agree to follow the Code of Conduct.
• I have searched for existing issues that already include this feature request, without success.

Describe the Feature Request

It would be useful to get shadow parts exposed for styling the header section of the ion-datetime component.

Currently, there is a title slot, but this does not give full control over the look of the datetime header. Most importantly, changing the background color.

for example:

ion-datetime { --header-background: ###### }

Describe the Use Case

This would solve the issue of being able to customize the datetime header

Describe Preferred Solution

No response

Describe Alternatives

The only alternative that really works is to do some hacky DOM manipulation after rendering like described here: https://forum.ionicframework.com/t/easy-way-to-style-the-ionic-datetime-header/236402

Related Code

No response

Additional Information

No response