add new attach API command dumpheapext for passing more options to jvm

The current implement of jcmd - jmap command could only pass less than 4 arguments to jvm.
With the introduction of "parallel=[N]" option, the "jmap -dump" option could have 4 arguments: "file=" , "-live", "gz=" and "parallel=".
To avoid the compatibility issue between old/new jmap and old/new jvm, a new attach API command "dumpheapext" needs to be added for passing extra arguments.