Add CBOR-friendly bulk format

The current bulk format in JSON uses the \n as end-of-line markers, allowing the coordinating node to parse just the metadata line, then skip over the body line.

CBOR doesn't have any disallowed characters or character sequences, so this mechanism will not work. Instead we should have a CBOR specific format which looks like this:

{ metadata line }
int_length_of_body_line
{ body line }

The int_length_of_body_line would allow us to skip to the next metadata line.

For the delete action, no int_length_of_body_line would be required, as we already know that it can't have a body.

Depends on #7640