lib: os: cbprintf: Add function for converting package

Extend package copying functionality by adding function for converting
a package. Function gets callback+context pair and converted package
is part by part passed to that callback. Contrary to typical sprintf
callback which works on chars, callback works with buffers.

Existing cbprintf_package_copy function is implemented as static
inline and uses new cbprintf_package_convert API.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>