edk2_initialize: add edk2_initialize

Add the edk2_initialize invocable, which replaces edk2_setup and
edk2_ci_setup. stuart_init is able to initialize both repositories and
submodules.

Integration instructions:
* import from edk2toolext.invocables.edk2_initialize rather than
  .edk2_setup or .edk2_ci_setup inside the configuration file
* import Submodule and/or Repository instead of RequiredSubmodule
* Where the parent class is specified, switch from SetupSettingsManager
  or CiSetupSettingsManager to InitializeSettingsManager
* Use get_required_repositories and get_required_submodules instead of
  GetDependencies and GetRequiredSubmodules respectively
* Use Submodule instead of RequiredSubmodule and Repository instead of
  a dictionary.