Provide a way to customize the ObjectMapper used in AMCB

[sobychacko]

AbstractMessageChannelBinder uses a custom ObjectMapper. It has code to check if the application provides an ObjectMapper. However, since AMCB is in a binder child context, it doesn't see beans provided by the application, which is only available in the parent context. See this for more context. Consider a way for the applications to circumvent this limitation.

[olegz]

Why do we need it? I mean why do we need to expose its customisation?