Fixes https://github.com/spring-cloud/stream-applications/issues/275
Many functions make use of `ComponentCustomizer` ot let end-user
to provide any custom configuration for target component used by the function.
First of all for use-case which are not covered by standard configuration
properties and secondly for those options which cannot be represented
as a configuration option.
* Add a sentence to READMEs of those functions which make use of `ComponentCustomizer`
with an expected generic argument type
* Add README into `rabbit-supplier` where it is fully missed at the moment
35a37fac5a