4062 [integration_test] Update README installation instructions (cla: yes, waiting for tree to go green, p: integration_test)

4063 [in_app_purchase] Added BillingClient.isFeatureSupported (cla: yes, p: in_app_purchase, platform-android)

4064 [flutter_plugin_tool] Refactor createFakePlugin (cla: yes, waiting for tree to go green)

4065 Update tool commands (cla: yes)

4066 [flutter_plugin_tools] Simplify extraFiles in test utils (cla: yes)

4067 [flutter_plugin_tools] Add a new base command for looping over packages (cla: yes)

4068 [flutter_plugin_tools] publish-plugin check against pub to determine if a release should happen (cla: yes)