Only fetch external entities that have changed since last sync

[sdewitt-newrelic]

Feature Description

When fetching external entities, only entities that have changed since the last run of the Syncer should be returned.

Describe Alternatives

None

Additional context

This is an efficiency feature. Calls to the provider backend may be expensive in terms of consumed resources, especially as the data set of external entities and/or the number of key-value pairs being retrieved increase. Adding a way to only get the set of entities that have changed since the last run of the Syncer mitigates this problem.

Not all providers may support this but a generic way to pass it should be added. Providers that do support it should honor it when passed.

[sdewitt-newrelic]

Completed with #12