[FEATURE] Make Thread Pool available to Process Node

[dbwiddis]

Is your feature request related to a problem?

The ProcessNode.execute() method introduced in #26 executes a workflow step asynchronously. This execution should share resources with the OpenSearch thread pool.

What solution would you like?

PR #38 will be adding the create components extension point which includes the Thread Pool. This pool needs to be accessible in some way to the Process Node class.

Presently the TemplateParser class uses static methods, but this class can probably be turned into a singleton, instantiated as part of the createComponents method execution with a copy of the thread pool, that it can pass on to the ProcessNode in its constructor.

What alternatives have you considered?

There may be other ways to pass around the threadpool in a static or singleton class somewhere.

Do you have any additional context?

Using the OpenSearch Thread Pool is pretty much mandatory for properly sharing resources and tracking thread stats / hot threads, etc.