[FEATURE] Add getExecutorBuilders extension point to SDK BaseExtension interface

[joshpalis]

Is your feature request related to a problem?

In the AnomalyDetectorJobRunner, when the job runner's runJob() method is invoked by Job Scheduler, AD creates a Runnable here that executes the job when invoked. The runnable is then submitted to this threadpool executor here for the threadpool named ad-threadpool.

The executor builder for this particular threadpool is instantiated here within the getExecutorBuilders extension point.

Within node.java, the executor builders for each plugin is retrieved here, which is then used to instantiate the threadpool here.

What solution would you like?

The SDK's BaseExtension interface should have a method called getExecutorBuilders in which we return a list of ExecutorBuilders from an extension. These executor builders should then be retrieved from the extension during ExtensionsRunner initialization and used to instantiate the threadpool here.