Allow CharArrays to be created from a BytesReference

[jkakavas]

Copying from #43197 (comment) :

.. it seems like most of the places we use CharArrays like this we have a BytesReference, and we end up copying to a byte array in order to convert to chars. I wonder if we could flip the CharArrays input around so it takes a BytesReference, so that we don't need to have the try/finally like this. In the few cases we actually have a real bytes array, we can wrap the array in a bytesreference.

[elasticmachine]

Pinging @elastic/es-core-infra

[williamrandolph]

This is a small refactor and I think we'll be able to see from a PR whether it's still a good idea or not.

[williamrandolph]

It turns out that it's not quite trivial because CharArrays is in the libs:core project while BytesReference is in elasticsearch:server. There are 12 usages of the CharArrays#utf8BytesToChars method.

@rjernst Do you think this is still worth doing?

[rjernst]

I think it would be difficult to move BytesReference to libs:core, partly because it would create a circular dependency with libs:x-content because BytesReference implements TXContentFragment. So I think we can safely close this as infeasible.