Generate migration and request spec

What does this MR do and why?

It adds a generator that adds a migration with a new member_roles column and a request spec file.

How to set up and validate locally

Run rails g gitlab:custom_roles:code --ability PERMISSION_NAME and check the generated files

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.

Closes #434351 (closed)