feat(flags): add flag series to event graph

[michellewzhang]

adds release-like lines for feature flag changes on top of the event bar chart. there is a legend to toggle the feature flag lines on and off.

new legend to match the figma:

the video below shows the graph with mock data with the 3 types of possible flag actions - created, updated, deleted:

https://github.com/user-attachments/assets/62aeb0f5-9908-411c-ac99-e8841f7ec1c3

With real data returned from the endpoint:

https://github.com/user-attachments/assets/b77b33e2-2842-4a58-8c0a-bf25d2d93be7

closes https://github.com/getsentry/sentry/issues/77812