Add instance level job token application settings

Related to: #440697 (closed)

What does this MR do and why?

Introduces instance settings for job token scope to toggle enforcement of allowlist for all projects.

• When on (default on %18.0): all projects will enforce their job token scope allowlist even if the scope is disabled at instance setting. If allowlist is empty, only job tokens from the current project are allowed.
• When off: only projects that have the scope enabled in the project settings will enforce the allowlist. Other projects will allow authentications from CI_JOB_TOKENs from other projects.

FE-related changes will be done in follow-ups MRs, issues

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Before	After

How to set up and validate locally

Switch to this branch

Run migrations: bin/rails db:migrate

In the Rails console, check that accessing the field does not throw an error:

[1] pry(main)> ::Gitlab::CurrentSettings.enforce_ci_inbound_job_token_scope_enabled = true
=> true