Add new project setting to display pipeline variables

What does this MR do and why?

This is the backend work necessary to add a project setting ci_display_pipeline_variables and the corresponding GQL work necessary to update it.

Even though there are a lot of files in this change, most of them are one liners just setting things up. Changes include:

• Add migration to add setting:

ci: == [advisory_lock_connection] object_id: 146520, pg_backend_pid: 45580
ci: == 20250804161406 AddDisplayPipelineVariablesToProjectCiCdSettings: migrating =
ci: -- add_column(:project_ci_cd_settings, :display_pipeline_variables, :boolean, {:default=>false, :null=>false})
ci:    -> 0.0157s
ci: == 20250804161406 AddDisplayPipelineVariablesToProjectCiCdSettings: migrated (0.0251s)

ci: == [advisory_lock_connection] object_id: 146520, pg_backend_pid: 45580

• GraphQL: Add argument to the CiCdSettingsUpdate mutation
• GraphQL: Add actual setting to CiCdSettingType
• Add boolean method and setting, plus getter and setter\

FE bit: Adds FE for ui setting** -> to be merged after this MR.**

References

#505440 (closed)

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.