WITH AUTH_SOCK PLUGIN USER

Description: On client side, if server's default plugin is
             different than that of client, client discards
             packet containing scramble information. This
             means that if server has default plugin
             caching_sha2_password and client has default
             plugin mysql_native_password, and if client is
             trying to connect to server using a user with
             plugin C (in this case auth_socket), following
             will happen:

             1. Client will discard scramble data
             2. Client will call native_password's client
                side authentication plugin
             3. In client_mpvio_read_packet, client will
                send user details to server and wait for
                scramble
             4. Server, having received user details, goes
                 on to process client reply and finds that
                 there are 3 plugins involved. It then
                 triggers a RESTART of authentication on
                 server side without sending anything to
                 client. As a part of restart, server uses
                 user's actual plugin (auth_socket) and
                 calls authenticate API for the same.
             5. auth_socket plugin, having received user
                details and connection info, performs
                verification and sends OK/ERROR.
             6. On client side, since client expects random
                data of length 20 from server, native
                plugin's authentication API will report
                error upon receing OK/ERROR.
             7. run_plugin_auth() won't find expected reply
                (because OK/ERROR was already read) and
                exit with error.

Solution: In run_plugin_auth(), there is a check for
          auth-switch packet. Fix is to extend it to cover
          OK packet too.

(cherry picked from commit b5840b451966469f4b527ba062147897750258cf)