Bluetooth: Audio: Shell: Replace hacky metadata handling

The hacky handle_metadata_update that copied the metadata to
a variable, since stream->codec->meta could not be modified,
has been replaced with a proper solution now that
stream->codec->meta can be modified.

It still only supports settings the streaming context, but
it is much easier to expand now.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>