feat: add user name argument to `auth_method` api

User name is usually used as a salt for user password in order to
avoid password repeating.
For instance, postgres md5 authentication stores passwords as
md5("password", "user"), so that the same passwords are represented by
different hashes.

part of picodata/picodata/sbroad!377

@TarantoolBot document
Title: Document updated `box.schema.user.password` declaration.

Since auth methods can use user name for hashing, user name is
added to argument list of `box.schema.user.password`.

NO_TEST=there are no methods that use user name