JIRA: https://jira.spring.io/browse/INT-4160
* Rename `Scripts.script()` to processor() for better context
* Add `Scripts.messageSource()` for the `ScriptMessageSourceSpec`
* Redo `DslScriptExecutingMessageProcessor` logic to deal with `GroovyScriptExecutingMessageProcessor` via reflection, since that class is a part of `SI-Groovy` module
* Upgrade to `JRuby-9.1.5.0`
* Get rid of reflection in the `DslScriptExecutingMessageProcessor`
* Introduce `ScriptExecutingProcessorFactory` and its `GroovyAwareScriptExecutingProcessorFactory` extension in the `groovy` module
* Register `GroovyAwareScriptExecutingProcessorFactory` as a bean via `GroovyIntegrationConfigurationInitializer`
* Use bean for `ScriptExecutingProcessorFactory.BEAN_NAME` in the `DslScriptExecutingMessageProcessor` to populate `delegate` and fallback to the regular `ScriptExecutingMessageProcessor` instantiation if there is no such a `ScriptExecutingProcessorFactory.BEAN_NAME` bean
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