INT-1362: #request SpEL function support

* Add `RequestAttributesUtils` to encapsulate #request SpEL function logic
* Introduce `IntegrationRegistration` strategy
to separate beans registration responsibility by modules
* Add 'component scan' for `IntegrationRegistration` implementations
* Add `IntegrationRegistrationBeanFactoryPostProcessor` to call
beans registration logic on each `IntegrationRegistration` implementation

JIRA: https://jira.springsource.org/browse/INT-1362, https://jira.springsource.org/browse/INT-2469