Expand subscription data source to support other web protocols such as gRPC, etc.

Expected Behavior

When adding custom data from the web, we are limited to rest calls. I would expect to be able to define our own stream reader that handles the communication.

Actual Behavior

You can only choose from an enum that creates the respective stream reader later on in the engine.

Potential Solution

Instead of using an enum. Change Subscription data source to an abstract class. The new abstract class will take a Func that returns a stream reader that will be called in the engine to match the current behavior of stream reader initialization in the engine. Derived classes will be created to create cleaner constructors. Derived classes are LocalFileSubscriptionDataSource, RemoteFileSubscriptionDataSource, WebSubscriptionDataSource, and RestSubscriptionDataSource. RestSubscriptionDataSource is derived from WebSubscriptionDataSource. WebSubscriptionDataSource should be used for any additional web protocols a person would want to use.

Checklist

• I have completely filled out this template
• I have confirmed that this issue exists on the current master branch
• I have confirmed that this is not a duplicate issue by searching issues

• I have provided detailed steps to reproduce the issue

Start by locating the subscription data source enum and the engine code that initializes its stream reader. Compare the proposed LocalFileSubscriptionDataSource, RemoteFileSubscriptionDataSource, WebSubscriptionDataSource, and RestSubscriptionDataSource hierarchy, then verify that a caller can provide a custom reader for protocols such as gRPC.