text: Title: [Alerting] alerts client find method returns partial alerts, but says it returns full alerts
  Body: Currently, the alertsClient `find()` API supports a `fields` option which allows callers to pick which fields they want returned in the alert.  So, the result can essentially be a partial alert, not a "full" alert.  However the signature on the method indicates it returns full alerts.
  
  It seems like we should do _something_ here, some thoughts on what that would be:
  
  - change the result of the `find()` to be a `Partial<SanitizedAlert[]>` instead of `SanitizedAlert[]`
  - remove the `fields` parameter, so you always get a full alert
  
  The second wouldn't make sense if the underlying ES call doesn't actually return fields that aren't specified in the fields parameter, but I'm guessing it does (perhaps the SO client does that filtering as well??).  I _think_ the result we're getting back from the SO is essentially the full alert (the `_source` field), so the only reason to filter it out in the API is to reduce memory / network usage for the caller.  And I'm guessing there's not much savings there.  If that's true, just removing the ability to filter with `fields` seems like it would make sense, and probably makes it simpler for clients, since otherwise they would to deal with a `Partial<Alert>`.
  
  Some relevant snippets:
  
  https://github.com/elastic/kibana/blob/d932830218c00c6548957c0954fcc4ae550352cb/x-pack/plugins/alerts/server/alerts_client.ts#L84-L98
  https://github.com/elastic/kibana/blob/d932830218c00c6548957c0954fcc4ae550352cb/x-pack/plugins/alerts/server/alerts_client.ts#L104-L109
  https://github.com/elastic/kibana/blob/d932830218c00c6548957c0954fcc4ae550352cb/x-pack/plugins/alerts/common/alert.ts#L49
  https://github.com/elastic/kibana/blob/d932830218c00c6548957c0954fcc4ae550352cb/x-pack/plugins/alerts/common/alert.ts#L27-L47
  
  finally, here's the code in `find()` that deletes properties not in `fields` - note the cast to `RawAlert` of the `pick()` result:
  https://github.com/elastic/kibana/blob/d932830218c00c6548957c0954fcc4ae550352cb/x-pack/plugins/alerts/server/alerts_client.ts#L356-L364
  