show: indicate length, encoding of omitted body content

If a leaf part's body content is omitted, return the encoded length and
transfer encoding in --format=json output.  This information may be used
by the consumer, e.g. to decide whether to download a large attachment
over a slow link.

Returning the _encoded_ content length is more efficient than returning
the _decoded_ content length.  Returning the transfer encoding allows
the consumer to estimate the decoded content length.