Add an API to search for the frequency supported by a timer that
is closest to the given target frequency.

This is in fact non-trivial to get right, as pre-scaler registers can
be 16 bit or even 32 bit in size, making a naive loop over all possible
pre-scalers too expensive (computationally).

This makes use of the `periph_timer_query_freqs` feature:

1. It does choose the closest frequency supported before calling
   timer_init() in the ztimer_periph_timer backend.
2. It does make use of the actually chosen frequency when using
   `ztimer_convert_frac`.
3. It does `assert()` the frequency is within 5% of the specified when
   no frequency conversion is performed or `ztimer_convert_shift_up`
   is used.