GH-275: Mention `ComponentCustomizer` in READMEs

[artembilan]

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

[artembilan]

While working on this I asked myself why not every function has this customizer hook? Did we make just a sane choice only for those where not everything can be covered with confit props? Is there any other idea?

[onobc]

While working on this I asked myself why not every function has this customizer hook? Did we make just a sane choice only for those where not everything can be covered with confit props? Is there any other idea?

Yep - sanity. Your exact comment from PR was:

Some modules just does not make sense to modify for more advance customization since all their configuration is covered with plain configuration properties. See splitter-function or wavefront-consumer

[artembilan]

Closed in favor of: https://github.com/spring-cloud/stream-applications/pull/467