[Response Ops][Alerting] Restructure actions FT route definition

Suggestion from PR review for https://github.com/elastic/kibana/pull/178682 to restructure the way routes are defined in the actions FT plugin. Referencing the route definitions in x-pack/test/alerting_api_integration/common/plugins/actions_simulators/server/unsecured_actions_simulation.ts

should we structure this like we do pagerduty, so we don't hardcode the `/api/...` path - which in theory, someday, could become a real API (but doubtful).  Eg, 

- https://github.com/elastic/kibana/blob/main/x-pack/test/alerting_api_integration/common/plugins/actions_simulators/server/pagerduty_simulation.ts
- https://github.com/elastic/kibana/blob/main/x-pack/test/alerting_api_integration/common/plugins/actions_simulators/server/plugin.ts

This is mostly pre-existing code, so I'd say we could just create a cleanup issue for it as well ...

Originally posted by @pmuellr in https://github.com/elastic/kibana/pull/178682#discussion_r1532077936