net.box: allow to specify authentication method

This commit adds a new option for net.box.connect - auth_type.
The option takes an authentication method name (string) that will
be used to authenticate the client on connect. Like user name and
password, the new option may also be passed via uri parameters, e.g.

  net.connect('host:port', {
      user = 'user',
      password = 'password',
      auth_type = 'chap-sha1',
  })
  net.connect('user:password@host:port?auth_type=chap-sha1')

The only authentication method supported by Community Edition (CE)
is 'chap-sha1' so we don't document or announce this feature in CE.
More methods and tests will be added to Enterprise Edition (EE).

Part of #7988

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