Contextual operators: transformWithWrappers / FunctionalWrappers

[simonbasle]

This PR introduces several concepts:

• a generic FunctionalWrappers interface that defines wrappers for all functional types used by Reactor API
• a contextual implementation of the above in ContextPropagation
• an operator that will allow transformation of a sequence with a FunctionalWrappers instance in scope, which enables users to wrap their lambdas inside the scope and get context propagation if available

See tests for an example of using Flux#transformWithWrappers.

[simonbasle]

Depends-on: #3145

[chemicL]

Closing. This work was superseded by proposal (4) from #3149.