Update for Runtime v100.0

[nixta]

Clustering is not yet available in the new Runtime v100.

Update this plugin to work with Runtime v100

• [x] Investigate performance of using AGSFeatureCollectionTable with AGSFeatureLayer. Update: Performance is good. Going ahead with implementation.

Tasks:

• [x] Implement clustering engine.
• [x] Implement cluster representation.
• [ ] Bring work into v.next branch.
• [ ] Implement unclustered feature representation.
• [ ] Generically handle any spatial reference.
• [ ] Calculate grid cell sizes properly.