Support functions for URL generation

[Tishka17]

Sometimes URL is constructed in more complicated way than just string interpolation. We can support it by allowing user to pass function as URL generator.

E.g

def url_generator(xxx: str):
    if not xxx:
       return "/me"
    return f"/users/{xxx}"

@get(url_generator)
def foo(self, xxx: str) ->Result:
   pass