Restructured and moved most APIs under frappe.core.api.file namespace.
Changed some obvious security gaps (like using get_list instead of
get_all for an endpoint), styled, added type hints and made minor performance
enhancements.
Changes
* download_file API
* Move API to handler.py
* Check for permissions via File.is_downloadable instead
* Moved APIs to new namespace: `frappe.core.api.file`
* Backwards compatibility
* Added APIs to override_whitelisted_methods to maintain existing
client endpoints
* Imported APIs to controller's namespace to avoid breaking external
app usages
