Create service_access_tokens table

What does this MR do and why?

Introduce a table to store JWT tokens for Code Suggestions on SM instances.

In this MR, the table & the model is no-op, we just preparing the ground via small MRs.

See #416996 (closed)

How to set up and validate locally

Run the migration.

In rails c, check that the table exists and is accessible via the model:

[1] pry(main)> ::Ai::ServiceAccessToken.count
  Ai::ServiceAccessToken Count (0.5ms)  SELECT COUNT(*) FROM "service_access_tokens" /*application:console,db_config_name:main,console_hostname:Alekseis-MBP-2.home,console_username:al,line:(pry):1:in `__pry__'*/
=> 0
[2] pry(main)>

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 #416996 (closed)