Implementation of Dynamic Plugins

[radiantgurl]

Depends on #20975, #21239 and #20298

Objective

• Implements a way to have dynamically installable and removable plugins for Bevy

Solution

• Implements two new functions on the Plugin trait: fn is_dynamic(&self) -> bool and fn remove(&self, app: &mut App)

Testing

• Not tested at the moment

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[alice-i-cecile]

Quick review thoughts from Discord:

[21:24]Alice 🌹
: Generally quite reluctant to add any features / encouragement to use subapps frankly
[21:25]Alice 🌹
: They are a major area of tech debt, and I have been unimpressed with the core architecture around them
[21:25]Alice 🌹
: Similarly for adding more features / complexity to plugins
[21:26]Alice 🌹
: I'm pleased that there's an example, but the example still doesn't motivate "why" you would ever want to modify a subapp at runtime