Add command creation callback

[Bodix]

There is a problem with the initialization order when using dependency injection (in my case, Zenject). The problem is that the OnCommandsCreated callback is called too late for Zenject. Some internal logic is executed before it (namely, the command variable change callbacks registered to the OnValueChanged attribute), which may require dependencies that Zenject should have provided. This causes a NullReferenceException.

Initialization order of command:

1. Initializing of default values of variables.
2. Loading variables values from PlayerPrefs if they exist.
3. Changing variables values followed by a callback. This may cause a NullReferenceException because
   the command dependencies have not yet been injected.
4. Injecting dependencies by Zenject (too late).

So I created the OnCommandCreated callback, which is invoked per command before any internal callback.

Also, I recommend renaming the previous OnCommandsCreated to OnCommandsInitialized to distinguish between creation and initialization. Same with the HasCommandsCreated property (to HasCommandsInitialized). I didn't do it because it is breaking changes, and I don't know your package update pipeline.