Adds a --sparse option that will skip caching app_store bundles to keep baked configs smaller.

Ensures that when an engine is flagged to be skipped in the caching process, the apps in the engine are still checked.
Previously, the engine's apps were not checked when the engine was skipped.

Adds an additional parameter to cache_apps() which will be used to check the descriptor type and determine whether to cache the bundle or not. Defaults to using the internal _should_skip_caching() function if not provided.

- some additional comment/docstring updates

- Adds a --sparse option that will skip caching any bundles with app_store descriptors in order to keep the bundle_cache smaller

- Fixes a bug in tk-core that would skip checking if an engine's apps needed to be cached if the parent engine was skipped.

- Adds a new parameter to cache_apps() in tk-core. The param "should_skip_caching_callable" accepts a callable which can return True or False, indicating whether caching should be skipped for the bundle. If the parameter isn't provided, cache_apps() falls back on the check built-in to core.