[Run] Renamed decorator and made it modular

[guy1992l]

• The decorator was renamed to handler - function is a reserved keyword in python's builtin, and even if it wasn't the decorator do not magically turns the function into MLRun function. mlrun.handler is more accurate to the usage, as to run the function from an MLRun function, you give the function's name as a handler.
• Auto-logging artifacts types - When logging types are not given by the user, the decorator will now log the returned object by its type according to a default map.
• Modular and expandable - The decorator can now be enriched with further logging / parsing functions.

[Tankilevitch]

Looks de best!