Add ObservationApplier abstraction

[ilya40umov]

This PR is trying to address the problem described in gh-4827.

• Introduce ObservationApplier and ObservationApplierRegistry abstractions
• Create an implementation of ObservationApplier for CompletionStage
• Create an implementation of ObservationApplier that can be used on suspending methods

Cons:

• ObservationToSuspendingMethodApplier will only work correctly when Spring Framework is in use (due the implementation taking advantage of how Spring AOP is handling calls to suspending functions)
  • Due to this, I'm not quite sure how to write tests for this class at the moment
• ObservationToSuspendingMethodApplier needs to refer to Mono, and so io.projectreactor:reactor-core is needed on the classpath
• ObservationToSuspendingMethodApplier is not registered automatically (could be done at the service startup by Spring Boot etc.)

Not done yet:

• Javadocs
• Tests

Open Questions:

• Given the Cons mentioned above, should ObservationToSuspendingMethodApplier potentially be moved to Spring Framework or Spring Boot instead?