support for IntelliJ dynamic plugins

[devoncarew]

We should add support to the Flutter plugin for IntelliJ's new dynamic plugins:

https://www.jetbrains.org/intellij/sdk/docs/basics/plugin_structure/dynamic_plugins.html

I'm not sure of the scope of the work here.

[stevemessick]

I think we're gated on Android Studio supporting it. Or we could split IntelliJ and Android Studio plugin into separate distros.

[stevemessick]

4.1 supports this. We should get started.

[stevemessick]

According to the JetBrains plugin verifier:

Plugin cannot be loaded/unloaded without IDE restart because it declares non-dynamic extensions: com.android.moduleDescriptionProvider, com.android.project.projectsystem
Plugin cannot be loaded/unloaded without IDE restart because it declares a group without 'id' specified

We will be able to delete the com.android.moduleDescriptionProvider when Android Studio 4.2 becomes the stable release.

We cannot delete the com.android.project.projectsystem as it is required to show the Project Structure menu. We'll have to rely on Android Studio making it dynamic.

Our pop-menus have <group> with no id but I think that will be easy to change.

We should use com.intellij.ide.plugins.DynamicPluginListener to either prevent reloading while an app is running, or ask the user if it is OK to restart it.

Most of the plugin downloads are for Android Studio. We'll keep this on hold until Android Studio has made that extension dynamic.

[stevemessick]

Also saw this in the log file:
plugin io.flutter is not unload-safe because of extension to non-dynamic EP com.intellij.defaultLiveTemplatesProvider

[stevemessick]

This work requires #6076 to be fixed.

The com.android.project.projectsystem EP is no longer required.

[stevemessick]

cc @jacob314