Add Code Suggestions token into AppSettings

What does this MR do and why?

Make Self-Managed configurable with admin's Personal Access Token (from SaaS) that will be used to auth for Code Suggestions and, potentially, other AI features.
This is done by an adding additional field into AppSettings.

This MR is a Backend part. Frontend changes will be in the next MR - !121565 (merged) (WIP)

No documentation changes for now, because the immediate goal of having this in prod is to simplify the PoC assembly. The UI change will be behind the ENV variable and is not expected to be used outside of PoC yet or visible by arbitrary SM instance users.

How to set up and validate locally

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

[19] pry(main)> ::Gitlab::CurrentSettings.ai_access_token
=> nil

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #412436 (closed)