egui_extras: Make serde an optional feature (#4641)

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All the other crates in egui have serde as an optional dependency -
which is great! But sadly egui_extras unconditionally includes it, which
adds a bunch of code to stuff that may not care for it. This PR gates
serde support behind a new `serde` feature.

This is a breaking change; if that's undesirable then we can add it as a
default feature instead, though that wouldn't match any of the other
crates.

crates/egui_extras/src/datepicker/popup.rs

@@ -6,7 +6,8 @@ use super::{button::DatePickerButtonState, month_data};
 
 use crate::{Column, Size, StripBuilder, TableBuilder};
 
-#[derive(Default, Clone, serde::Deserialize, serde::Serialize)]
+#[derive(Default, Clone)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 struct DatePickerPopupState {
     year: i32,
     month: u32,