Support MySQL Authentication Switch protocol.

[xiangyhu]

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Ensure you have provided the following details while reporting a problem:

• The exact version of the related software, including but not limited to the OpenResty version
  (if any), the NGINX core version, ngx_lua module version, this Lua library''s version,
  and operating system version.
  Any version

• A minimal and standalone test case that others can easily run on their side and
  reproduce the issue you are seeing.
  Connect with a MySQL user created with non-default authentication plugin.

• Do not simply say "something is broken" or "something does not work". Always provide
  as much details as possible. Always describe the symptoms and your expected results.
  MySQL has standard protocol of authentication switch flow as part of authentication and we would like OpenResty to support.
  https://dev.mysql.com/doc/internals/en/auth-method-switch.html

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