[keyserver/lib] Introduce a specialRoles enum and special_role column in MariaDB

Summary:
We introduce the specialRoles enum that maps a DEFAULT_ROLE to a specfic number we store in MariaDB. With this, we can run a migration that:

1. Adds a special_role column to roles
2. Update each row in roles to populate the special_role field if it is the defualt role

Addresses ENG-5831 and ENG-5832

Test Plan:
Ran the migration and confirmed it succeeded. Also checked my roles table to make sure the correct roles were updated

Reviewers: atul, ginsu, tomek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D9961