Add default implementation of Serialize and Deserialize to Timer and Stopwatch (#6248)

# Objective

Fixes #6244

## Solution

Uses derive to implement `Serialize` and `Deserialize` for `Timer` and `Stopwatch`

### Things to consider
- Should fields such as `finished` and `times_finished_this_tick` in `Timer` be serialized?
- Does `Countdown` and `PrintOnCompletionTimer` need to be serialized and deserialized?

## Changelog

Added `Serialize` and `Deserialize` implementations to `Timer` and `Stopwatch`, `Countdown`.

crates/bevy_internal/Cargo.toml

@@ -45,7 +45,7 @@ wav = ["bevy_audio/wav"]
 # Enable watching file system for asset hot reload
 filesystem_watcher = ["bevy_asset/filesystem_watcher"]
 
-serialize = ["bevy_input/serialize", "bevy_window/serialize"]
+serialize = ["bevy_input/serialize", "bevy_time/serialize", "bevy_window/serialize"]
 
 # Display server protocol support (X11 is enabled by default)
 wayland = ["bevy_winit/wayland"]

crates/bevy_time/Cargo.toml

@@ -8,6 +8,9 @@ repository = "https://github.com/bevyengine/bevy"
 license = "MIT OR Apache-2.0"
 keywords = ["bevy"]
 
+[features]
+default = []
+serialize = ["serde"]
 
 [dependencies]
 # bevy
@@ -18,3 +21,4 @@ bevy_utils = { path = "../bevy_utils", version = "0.9.0-dev" }
 
 # other
 crossbeam-channel = "0.5.0"
+serde = { version = "1", features = ["derive"], optional = true }

crates/bevy_time/src/stopwatch.rs

@@ -24,6 +24,7 @@ use bevy_utils::Duration;
 /// assert_eq!(stopwatch.elapsed_secs(), 0.0);
 /// ```
 #[derive(Clone, Debug, Default, Reflect)]
+#[cfg_attr(feature = "serialize", derive(serde::Deserialize, serde::Serialize))]
 #[reflect(Default)]
 pub struct Stopwatch {
     elapsed: Duration,

crates/bevy_time/src/timer.rs

@@ -10,6 +10,7 @@ use bevy_utils::Duration;
 ///
 /// Paused timers will not have elapsed time increased.
 #[derive(Clone, Debug, Default, Reflect)]
+#[cfg_attr(feature = "serialize", derive(serde::Deserialize, serde::Serialize))]
 #[reflect(Default)]
 pub struct Timer {
     stopwatch: Stopwatch,
@@ -401,6 +402,7 @@ impl Timer {
 
 /// Specifies [`Timer`] behavior.
 #[derive(Debug, Clone, Copy, Eq, PartialEq, Hash, Default, Reflect)]
+#[cfg_attr(feature = "serialize", derive(serde::Deserialize, serde::Serialize))]
 #[reflect(Default)]
 pub enum TimerMode {
     /// Run once and stop.