attach method
Sets up the plugin with the ContentExtensionBuilder. The plugin relies on the ContentExtensionBuilder to do its work.
Implementation
@override
void attach(ExtensionBuilder extBuilder) {
assert(extBuilder is ContentExtensionBuilder,
'''For the $runtimeType to work, there must be one $ContentExtensionBuilder in your extension builders.
However, you have provided a ${extBuilder.runtimeType}''');
_extensionBuilder = extBuilder as ContentExtensionBuilder;
}