introduce combination of "brackets" and "numbered" option on slices

[sGy1980de]

This PR introduces the combination of "brackets" and "numbered" option on slices to provide indexed brackets in the URL.

Given

p := struct{
   Values []string `url:"key,brackets,numbered"`
}{
  Values []string{"value0", "value1"},
}

Currently this combination is possible but would lead to something like
key[]0=value0&key[]1=value1

This behavior will be changed to something similar like this PHP method would create:
https://www.php.net/manual/en/function.http-build-query.php

The upper example would look like this:
key[0]=value0&key[1]=value1

While there is nothing wrong with the brackets only syntax, some APIs, e.g. LinkedIn Ads API simply insist on the indexed brackets syntax. I'm aware this can be created with maps, but still this might be a useful extension.

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[sGy1980de]

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