Overriding a runnable via ID

[krispya]

This idea came from @pietrovismara. An ID is currently only used for debug, but it also allows an alias for overriding.
This would be useful for mocking during tests, but also for ecosystem integration where you can override a runnable that is loaded by another library.

[akdjr]

Interestingly enough, I was debating changing the underlying dag to have nodes identified by an ID instead of the function itself. That change would allow ID based overriding.