Mark interfaces which are Druid extension points in code

[leventov]

Following #3889 (comment) it would be nice to have APIs which are sensitive to changes being designated in code e. g. with @DruidExtensionPoint annotation.

cc @himanshug

[drcrallen]

I thought that's what druid-api is for. Or more specifically, if you are writing extensions and using classes that are NOT in druid-api, then they might change from release to release.

[leventov]

BufferAggregator is not in druid-api, but @himanshug pointed out that many people extend it

[himanshug]

@drcrallen @leventov i don't think there is an exhaustive list of interfaces that are considered the extension points. however, things mentioned in http://druid.io/docs/0.9.2/development/modules.html are considered public extension points.

[leventov]

Fixed in #4433