Add CrossSiteEval with ModelController

[SYangster]

• Add cross site model eval workflow using ModelController
• Expose save_flmodel, load_flmodel, get_run_dir, get_app_dir functions for option to save and load models without using persistor/ model locator

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Quick tests passed locally by running ./runtest.sh.
• In-line docstrings updated.
• Documentation updated.