Add java.time overloads to RxJava APIs

[kluever]

E.g., for the TestScheduler API:

• TestScheduler.advanceTimeBy(long delayTime, TimeUnit unit), please add: advanceTimeBy(Duration)
• TestScheduler.advanceTimeTo(long delayTime, TimeUnit unit), please add: advanceTimeTo(Instant)
• TestScheduler.now(TimeUnit), please add: now() (returning an Instant)

There's probably many other APIs that would benefit from java.time-based overloads as well.

Thanks!

[akarnokd]

See #7266 (comment)