Add Helidon-Reactive Scrabble benchmark

[akarnokd]

Add the industry standard Shakespeare plays Scrabble benchmark to measure Helidon-Reactive.

Results (i7 8700, Windows 10 x64, Java 11.0.6):

ShakespearePlaysScrabbleWithHelidonReactiveOpt   sample  165  30,909 ± 0,136  ms/op
ShakespearePlaysScrabbleWithRxJava2FlowableOpt   sample  225  22,427 ± 0,049  ms/op
ShakespearePlaysScrabbleWithRxJava3FlowableOpt   sample  227  22,206 ± 0,259  ms/op
ShakespearePlaysScrabbleWithRxJava3ObservableOpt sample  235  21,365 ± 0,039  ms/op

Edit: Remeasured RxJavas with JMH 1.23.

What could be improved in Helidon:

• Adding Single.flatMapIterable
• Create dedicated Multi.max and Multi.sum implementations.
• Create dedicated implementation for chars().
• More optimized Single.get().