Create submodule for activation related `homebrew` options
Create submodule for activation related `homebrew` options

* Adds `homebrew.onActivation` submodule.
* Moves `homebrew.autoUpdate` to `homebrew.onActivation.autoUpdate`.
* Moves `homebrew.cleanup` to `homebrew.onActivation.clean`.
* Adds new option `homebrew.onActivation.upgrade`.

LnL7/nix-darwin