egui_extras: Make serde an optional feature (#4641)

<!--
Please read the "Making a PR" section of
[`CONTRIBUTING.md`](https://github.com/emilk/egui/blob/master/CONTRIBUTING.md)
before opening a Pull Request!

* Keep your PR:s small and focused.
* The PR title is what ends up in the changelog, so make it descriptive!
* If applicable, add a screenshot or gif.
* If it is a non-trivial addition, consider adding a demo for it to
`egui_demo_lib`, or a new example.
* Do NOT open PR:s from your `master` branch, as that makes it hard for
maintainers to add commits to your PR.
* Remember to run `cargo fmt` and `cargo clippy`.
* Open the PR as a draft until you have self-reviewed it and run
`./scripts/check.sh`.
* When you have addressed a PR comment, mark it as resolved.

Please be patient! I will review your PR, but my time is limited!
-->

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/table.rs

@@ -528,7 +528,8 @@ impl<'a> TableBuilder<'a> {
 
 // ----------------------------------------------------------------------------
 
-#[derive(Clone, Debug, serde::Deserialize, serde::Serialize)]
+#[derive(Clone, Debug)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 struct TableState {
     column_widths: Vec<f32>,
 }