Add egui::Sides shrink_left / shrink_right (#7295)

This allows contents (on one of the sides) in egui::Sides to shrink. 

* related https://github.com/rerun-io/rerun/issues/10494

tests/egui_tests/tests/test_sides.rs

@@ -0,0 +1,76 @@
+use egui::{TextWrapMode, Vec2, containers::Sides};
+use egui_kittest::{Harness, SnapshotResults};
+
+#[test]
+fn sides_container_tests() {
+    let mut results = SnapshotResults::new();
+
+    test_variants("default", |sides| sides, &mut results);
+
+    test_variants(
+        "shrink_left",
+        |sides| sides.shrink_left().truncate(),
+        &mut results,
+    );
+
+    test_variants(
+        "shrink_right",
+        |sides| sides.shrink_right().truncate(),
+        &mut results,
+    );
+
+    test_variants(
+        "wrap_left",
+        |sides| sides.shrink_left().wrap_mode(TextWrapMode::Wrap),
+        &mut results,
+    );
+
+    test_variants(
+        "wrap_right",
+        |sides| sides.shrink_right().wrap_mode(TextWrapMode::Wrap),
+        &mut results,
+    );
+}
+
+fn test_variants(
+    name: &str,
+    mut create_sides: impl FnMut(Sides) -> Sides,
+    results: &mut SnapshotResults,
+) {
+    for (variant_name, left_text, right_text, fit_contents) in [
+        ("short", "Left", "Right", false),
+        (
+            "long",
+            "Very long left content that should not fit.",
+            "Very long right text that should also not fit.",
+            false,
+        ),
+        ("short_fit_contents", "Left", "Right", true),
+        (
+            "long_fit_contents",
+            "Very long left content that should not fit.",
+            "Very long right text that should also not fit.",
+            true,
+        ),
+    ] {
+        let mut harness = Harness::builder()
+            .with_size(Vec2::new(400.0, 50.0))
+            .build_ui(|ui| {
+                create_sides(Sides::new()).show(
+                    ui,
+                    |left| {
+                        left.label(left_text);
+                    },
+                    |right| {
+                        right.label(right_text);
+                    },
+                );
+            });
+
+        if fit_contents {
+            harness.fit_contents();
+        }
+
+        results.add(harness.try_snapshot(&format!("sides/{name}_{variant_name}")));
+    }
+}