Add new configs to support inter AZ stage-to-stage routing

[tcai-stripe]

Context

We want to reduce cross AZ traffic during scalar stage to stage communication so adding the following two configs:

• mantis.taskexecutor.router.scalar-stage-to-stage to support custom stage to stage router, it will fall back to the default round robin router on instantiation failure
• mantis.availabilityZoneUtils.class to allow users to provide availability zone for each worker

Checklist

• ./gradlew build compiles code correctly
• Added new tests where applicable
• ./gradlew test passes all tests
• Extended README or added javadocs where applicable

[crioux-stripe]

This looks pretty straightforward and as though we could use it to reduce costs stage to stage. I wanted @Andyz26 to take a look as well considering the impact it could have on Netflix.

Though I like that we're defaulting to the old behavior so the base effect is no change for Netflix.

[tcai-stripe]

@Andyz26 could you please help review this when you get a chance? As @codyrioux mentioned earlier that this shouldn't have any impact on Netflix and we want to plugin our own router to reduce cross AZ traffic