feat: Add `auth_type` to box.schema.user.create()

Now it's possible to specify the desired authentication method during
user creation via `auth_type`, e.g.

```lua
box.schema.user.create('mickey', { auth_type = 'chap-sha1',
                                   password = 'foobar' })
```

Furthermore, authentication methods may now specify that they don't
require password to create stored authentication info. This is used
in LDAP authentication (`auth_type = 'ldap'`):

```lua
box.schema.user.create('mickey', { auth_type = 'ldap' })
```

NO_DOC=picodata internal patch
NO_CHANGELOG=picodata internal patch
NO_TEST=picodata internal patch