golang: Add support for @func(Class) and @func to be used over @property

@func(Class) can be used with regular and staticmethod/classmethod functions, but

    @func(MyClass)
    @property
    def my_property(self):
        ...

was not working so far.

-> Add support for that as well, because I hit the need for it during my work on wendelin.core monitoring and doing the implementation was relatively straightforward.

Please see individual patches for details.

Kirill

/cc @jerome, @levin.zimmermann