[ML-DataFrame] add sync api

[hendrikmuhs]

Feature Branch PR

Introduces the synchronization API and adds a implementation for time-based synchronization.

Time-based synchronization means we require the existence of a time field while new data arrives with this field set accordingly with no updates, no deletes.

The API with an example looks as follows:

"sync" : {
  "time" : {
   "field" : "@timestamp",
   "delay": "5m"
  }
}

Other synchronization methods - instead of time - can be plugged in later, that's why fields below sync are implemented as named writables.

This PR adds the API without an implementation.

[elasticmachine]

Pinging @elastic/ml-core

[hendrikmuhs]

run elasticsearch-ci/1