`egui_extra`'s `datepicker` feature incompatible with `eframe`'s `persistence` feature

**Describe the bug**
When you enable both `egui_extra`'s `datepicker` feature and `eframe`'s `persistence` feature, `egui_extra` fails to compile.

**To Reproduce**
Steps to reproduce the behavior:
1. `cargo new test && cd test`
2. `cargo add eframe --features persistence`
3. `cargo add egui_extras --features datepicker`
4. `cargo build`

**Expected behavior**
It should build, but it doesn't due to unsatisfied trait bounds.

```
error[E0277]: the trait bound `DatePickerButtonState: SerializableAny` is not satisfied
error[E0277]: the trait bound `DatePickerPopupState: SerializableAny` is not satisfied
```

Reproduce the failure with the listed cargo new, cargo add, and cargo build commands, enabling eframe's persistence and egui_extras' datepicker features. Use the compiler's DatePickerButtonState and DatePickerPopupState trait-bound errors as the entry point; done means the same combination builds successfully.

Written by the indexing model from the issue text.