add a read-only variant for HELLO
As discussed in https://github.com/antirez/redis/issues/7364, it is good to have a HELLO command variant, which does not switch the current proto version of a redis server. While `HELLO` will work, it introduced a certain difficulty on parsing options of the command. We will need to offset the index of authentication and setname option by -1. So 0 is marked a special version meaning non-switching. And we do not need to change the code much.
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