Add new TokenIssuer types

Part of shipping Create PoC to demo interfaces and data flow (#515279 - closed) into production.

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Extract the new TokenIssuer and Tokens types from !184800 (closed).

They should be production-ready i.e. covered with tests and used by DuoWorkflowService::Client behind a FF.

We should deliver this work in at least the following increments or MRs:

• Extract TokenIssuer types. This can happen without a FF. MR: !185969 (merged)
• Extract Tokens class + changes to GitlabSubscriptions::AddOn (requires groupprovision review). MR: !186833 (merged)
  • Should introduce FF for rolling this out to Duo Workflow