introduce the syntax

... IDENTIFIED { WITH | VIA }
      plugin [ { USING | AS } auth ]
 [ OR plugin [ { USING | AS } auth ]
 [ OR ... ]]

Server will try auth plugins in the specified order until the first
success. No protocol changes, server uses the existing "switch plugin"
packet.

The auth chain is stored in json as

  "auth_or":[{"plugin":"xxx","authentication_string":"yyy"},
             {},
             {"plugin":"foo","authentication_string":"bar"},
            ...],
  "plugin":"aaa", "authentication_string":"bbb"

Note:
* "auth_or" implies that there might be "auth_and" someday;
* one entry in the array is an empty object, meaning to take plugin/auth
  from the main json object. This preserves compatibility with
  the existing mysql.global_priv table and with the mysql.user view.
  This entry is preferrably a mysql_native_password plugin for a
  non-empty mysql.user.password column.

SET PASSWORD is supported and changes the password for the *first*
plugin in the chain that has a notion of a "password"