Add pluggable mechanism to support multiple reactive or non-reactive technologies

[pellse]

Currently the Assembler reactive and non-blocking support is backed by Project Reactor. We expose the Flux<T> type and internally leverage Project Reactor api with Flux and Mono.

Exposing different reactive and non-reactive stacks like:

• RxJava
• Akka Streams
• Pekko Streams
• Mutiny
• Kafka
• Stream (JDK)
• etc.

through a pluggable adapter would make the Assembler library useful to a broader audience.