[Task Manager] Introduce generic scheduling field in place of interval

[gmmorris]

We want to change Tasks such that they can handle a variety of different kinds of scheduling, with interval being the single implementation we currently provide.

We'll address this by doing the following:

1. The interval field will be marked as deprecated on TaskManager, but we'll continue to support it until 8.0.0
2. Along side interval we'll introduce a field called sked (up for discussion ;)) which can take any shaped object, and migrate all existing intervals into the shape: { interval: "10m" }. This will allow us to support future shapes such as { cron: "0 0 0 0 0" } or { "hourly" : { "minute" : 30 } } similar to watcher.
3. We will make this change in both TaskManager and Alerting (two separate PRs) for 7.6, to keep both APIs aligned, but Alerting will continue to rely on it's own internal implementation as moving to TM probably can't be done in time for 7.6.

[elasticmachine]

Pinging @elastic/kibana-stack-services (Team:Stack Services)

[gmmorris]

closed by #52727