Allow SUBDOC_MULTI_MUTATION to set XATTRs and whole document in single call

As part of MB-23522, KV exposes below api

SUBDOC_MULTI_MUTATION(key="my_doc",

    specs=[{opcode=CMD_SUBDOC_DICT_UPSERT, flags=NONE, path="", value="{\"field\":42}"},

           {opcode=CMD_SUBDOC_DICT_UPSERT, flags=XATTR_PATH, path="my_xattr", value="{\"seq\": 123}"}])

Eventing project would like to use it for handling recursive mutations(Apologies, we haven't been communicating much about what eventing is about with others, hope to do it soon. Matt I would be familiar with what eventing is trying to do)

Would be useful even if we get an early experimental api support from lcb.