Add progress notifications

:bulb: Idea

Add a progress notification, that shows a progress bar and can be continuously updated. This was previously requested in https://github.com/binwiederhier/ntfy/issues/44 but we did not have notification update mechanism in ntfy at the time. Now that its in place, this should be trivial to implement.

:computer: Target components

ntfy server: Add a progress and progress_max header (This likely does not require any change on server side) all client apps: Use the value of above 2 headers to show a progress bar