Factor out generation of user values to inheritable function

## Description

There is no easy way to edit the values sent to newly generated users from oauth_signup.

In some cases, the mapping from an oauth provider can be different.
* ex: login is something other than email

In other cases, there are additional fields in res_users added by a module
* ex: firstname and last name in `partner_firstname` from OCA

Since this module basically only adds an overwrite to the `_auth_oauth_signin()` function, factoring out the part which is most likely to be altered is a good alternative to copying the `_auth_oauth_signin()` function each time a module needs different values.

This factorization allows modules inheriting from `auth_oauth_signup` to alter values sent to the copy of Template User with needed changes which I described. This means smaller changes to the default behaviour and the ability to properly inherit (multiple times if needed) this module without losing needed behaviour.

Problem described here https://github.com/OCA/partner-contact/issues/30