Fix user schema override using custom fields

Steps to reproduce:

1. Follow the steps in the documentation for adding new fields to the user profile in LTS 2021 
2. After server startup, observe following error in server logging at first login (using embedded H2 database):

ERROR [DefaultNuxeoExceptionHandler] javax.servlet.ServletException: org.nuxeo.ecm.directory.DirectoryException: getEntry failed
...
Caused by: org.h2.jdbc.JdbcSQLSyntaxErrorException: Column "manager" not found; SQL statement:
SELECT "manager", "company", "tenantId", "firstName", "email", "username", "lastName" FROM "users" WHERE "username" = ? [42122-200]
...

Note: This answers thread describes a similar error using Postgres (previously working in LTS 2019), solved by manually adding the column to the users table (test with MongoDB pending). This has implications for authentication configs that require custom user fields populated by external providers (for example, SAML 2.0 authentication).