feat: automatically import Android plugins

[imhoffd]

This is an implementation of the Android Plugin Auto-registration Proposal.

TODO:

• Support adding additional classes during initialization (e.g. plugins from the app itself)
• Make it backwards compatible with @NativePlugin (?)
• Think about error handling when reflection fails

Internally, the CLI creates a capacitor.plugins.json file inside android/app/src/main/assets (alongside capacitor.config.json) which is meant to be shipped in the final Android bundled app.

This file contains a list of classpaths to the Capacitor plugins it finds during npx cap update android. It looks like this:

[
        {
                "pkg": "@capacitor/action-sheet",
                "classpath": "com.capacitorjs.plugins.actionsheet.ActionSheetPlugin"
        },

        ...
]

Then, in Java, we use Reflection to load classes via Class.forName().

This simplifies the app developer workflow because they no longer need to use add() to add the class names to the list. MainActivity.java in most apps can look like this:

public class MainActivity extends BridgeActivity { }

If they want to add app plugins, they need to do something like this:

public class MainActivity extends BridgeActivity {
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    registerPlugin(CustomPlugin.class);
  }
}

depends on #3866
depends on #3762
resolves #3266