bin/show: Add --print-sizes option to show sizes metadata

Use the new `ostree_commit_get_object_sizes()` API to read the
`ostree.sizes` commit metadata and print a summary.

man/ostree-show.xml

@@ -99,6 +99,17 @@ Boston, MA 02111-1307, USA.
                 </para></listitem>
             </varlistentry>
 
+            <varlistentry>
+                <term><option>--print-sizes</option></term>
+
+                <listitem><para>
+                    Show the commit size metadata. This in only supported for
+                    commits that contain <varname>ostree.sizes</varname>
+                    metadata. This can be included when creating commits with
+                    <command>ostree commit --generate-sizes</command>.
+                </para></listitem>
+            </varlistentry>
+
             <varlistentry>
                 <term><option>--raw</option></term>