Allow multiple `ext_func`s in `Log::Filter`

[J-Gras]

As far as I can tell the current design allows only a single ext_func per Log::Filter. This may result in a conflict if multiple packages try to use this feature (e.g., https://github.com/J-Gras/add-interfaces/issues/2). Hence, it would be nice to allow multiple ext_funcs per Log::Filter.