Add mutation of xattrs

INSERT, UPSERT  options take xattrs and mutate

INSERT/UPSERT INTO airport (KEY     UUID(),

                     VALUE   {"a":1},

                     OPTIONS {"expiration": 2*60*60, "xattrs":{"x1":"x123", "x2":{"b:1}}});

UPDATE See if we can do same like meta().expiration convert subdoc mutation? (If SUBDOC API allows xattr's mutation with full document mutation because UPDATE is full document update and xattrs must use subdoc API and all must be done in one mutation) . Yes. From kv team ( subdoc “multi-mutation” is generally the approach to use.  A single mutation has multiple operations attached. First are the subdoc operations and finally is a single mutation to set the “value”/full-doc)