Add decorator for extending a native generic function

[jtuyls]

Decorator API for adding a new specialization of a native generic function for a specific target. This is for example useful for adding target specific strategies.

@tvm.target.extend_native_generic_func("add_strategy", "mytarget")
def add_strategy_mytarget(attrs, inputs, out_type, target):
    # Adds a specialization of the addition strategy for 'mytarget'
    ...

[tqchen]

close due to inactive status, feel free to reopen a new pr