This change cleans up some methods in the CharArrays class from x-pack, which
includes the unification of char[] to utf8 and utf8 to char[] conversions that
intentionally do not use strings. There was previously an implementation in
x-pack and in the reloading of secure settings. The method from the reloading
of secure settings was adopted as it handled more scenarios related to the
backing byte and char buffers that were used to perform the conversions. The
cleaned up class is moved into libs/core to allow it to be used by requests
that will be migrated to the high level rest client.

Relates elastic#32332