Create new Task Manager plugin APIs focused on ad-hoc and recurring tasks

[mikecote]

We should create dedicated ad-hoc and recurring task manager plugin APIs that are simple for developers to use. We should research what other scheduling and queueing system name their APIs and settle on naming with the team before implementing.

Examples:

• Deprecate schedule API in favour of a new createRecurringTask() and createAdHocTask() API. The API can take a parameter to determine if it should throw if the task already exists.
• Deprecate bulkSchedule API in favour of a new createRecurringTaskBulk() and createAdHocTaskBulk() API

Using the same prefix can make it easier to navigate the APIs in an IDE using a typeahead. Splitting the APIs between AdHoc and Recurring will allow us to make explicit task shapes for each (ex: no schedule property for ad-hoc tasks, etc)

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)