[SLO] Add new settings.syncField in UI and API

[kdelemme]

Summary

settings.syncField should be used to configure and override the default sync_field of the SLO rollup transform. We currently use the indicator.params.timestampField for the transform sync_field. This is not optimal for data that is ingested long after they happened, e.g. @timestamp >>> event.ingested

Acceptance criteria

• [ ] field is configurable from the API
• [ ] field is configurable from the form
• [ ] field is hidden behind a advanced settings section
• [ ] validation: field is a date and exists on the source index pattern

[elasticmachine]

Pinging @elastic/obs-ux-management-team (Team:obs-ux-management)