Add trigger methods for synchronization

[sallyom]

Currently, the only method for TriggerSpec is a cronspec string. A webhook endpoint trigger and/or a kubeapi trigger will provide more flexibility. With these methods, an on-demand synchronization will be enabled.

[JohnStrunk]

Proposal for the API version:

spec:
  trigger:
    manual: <int*>
status:
  observedManual: <int*>

if observedManual < manual then sync. At completion of sync, observedManual = manual.

So, by setting manual to a higher value, a sync gets triggered. Due to caching and failures, I'm not sure we can just have a simple != comparison. More thought is needed here.

[JohnStrunk]

Based on backube/scribe#153, can we close this?