Add ci_pipeline_variables_minimum_override_role project setting

This is currently used in conjunction with the toggle restrict_user_defined_variables to set the minimum role required to set pipeline and job variables, but may become a standalone setting in the future (restrict_user_defined_variables is redundant; it is the same as setting ci_pipeline_variables_minimum_override_role to developer).

Note for reviewers: This is my first contribution to the provider. I basically just copied what was done for ci_restrict_pipeline_cancellation_role.