Notification on request_id eligibility for a token

[JimTharioAmazon]

Send an event via EventBridge when request_ids become eligible for a token.
This will require some type of evaluation each time the serving counter is changed.
We may want to consider using EventBridge patterns and batching the notification when multiple request_ids are ready.

Listeners may want several filters on the events:

• All eligible request_ids
• Filters on one or more specific request_ids

Should this be related to #154 where another event is sent when a request_id's eligibility for a token has expired? This is because in many cases the window of time to obtain a token isn't open-ended and that available capacity can be used by another client in line.