Enhancement: Add mssql_ad_keytab_file and mssql_ad_keytab_remote_src variables to allow users to provide a pre-created keytab.

Reason: Sometimes users do not want the role to access a privileged AD account with adutil to create the keytab within the role, but instead they receive a pre-created keytab file that they want to input to mssql-server.

Result: Users can provide the keytab file with the mssql_ad_keytab_file variable according to an example playbook.

Acceptance Criteria:
It should be possible to pre-create a keytab file and then input it with the role using the variable mssql_ad_keytab_file. See how this is being tested in  https://github.com/linux-system-roles/mssql/blob/main/tests/playbooks/tests_ad_integration.yml#L85-L109