Add-on to support MDC

[ttddyy]

Motivation

When application gets certain size, MDC becomes one of the requirement for operational perspective.
I think it is an area lacking a support now.
People usually search how to achieve MDC and just copy paste the custom code into application.
Instead of proliferating such code, it would be very beneficial to have official MDC support.

Desired solution

Here is implementation ideas for MDC support module:

• Logging framework agnostic
  Spring has logic to auto detects major logging framework. Similar way would be nice to make the MDC support agnostic to user's choice of logging framework.

• Feature registration
  The MDC propagation logic can be registered to a hook or even bytecode manipulated when the feature is enabled.

Additional context

Relates: reactor/reactor-core#1985

[noah-iam]

I am still searching for solution to support MDC in reactive programming, and came to this PR.