Refine configsource interface (#2821)
Refine configsource interface (#2821)

* Add a few refinements to configsource interface

* Small re-wording.

* Remove function type.

* Remove WatchableRetrieved and add an error instead

* Add ErrWatcherNotSupported to WatchForUpdate function

* Fix typo
open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector-contrib,open-telemetry/opentelemetry-collector-contrib,open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector