This PR addresses a list of legacy code debt the plugin has incurred over the past year due to extensive changes in its internals and the adoption of the Kibana Platform.

It includes:
1. The `TaskManager` class has been split into several independent components: `TaskTypeDictionary`,  `TaskPollingLifecycle`,  `TaskScheduling`,  `Middleware`. This has made it easier to understand the roles of the different parts and makes it easier to plug them into the observability work.
2. The exposed `mocks` have been corrected to correctly express the Kibana Platform api
3. The lifecycle has been corrected to remove the need for  intermediary streames/promises which we're needed when we first introduced the `setup`/`start` lifecycle to support legacy.
4. The Logger mocks have been replaced with the platform's `coreMocks` implementation
5. The integration tests now test the plugin's actual public api (instead of the internals).
6. The Legacy Elasticsearch client has been replaced with the typed client in response to the deprecation notice.
7. Typing has been narrowed to prevent the `type` field from conflicting with the key in the `TaskDictionary`. This could have caused the displayed `type` on a task to differ from the `type` used in the Dictionary itself (this broke a test during refactoring and could have caused a bug in production code if left).