Create branching ARM LRO implementations

As follow-up from #4508, talking with @jhendrixMSFT we should probably create separate implementations much like Go has. While implausible, it's not impossible to effectively flip to another "final-state-via" type mid-flight.

I want to get #4508 in first and an ARM crate updated, but then will make the changes once we have all tests in place and a working example. Most services, at least, should work correctly with the initial implementation.