Add "target" parameter to DAP attach request

This adds a new "target" to the DAP attach request.  This is passed to
"target remote".  I thought "attach" made the most sense for this,
because in some sense gdb is attaching to a running process.  It's
worth noting that all DAP "attach" parameters are defined by the
implementation.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>