JIRA: https://jira.spring.io/browse/INT-4116

* Implement a `FileSplitter.FileMarker`-based aggregation strategies
and utilize them in a general `FileAggregator` component
* Make `HeaderAttributeCorrelationStrategy.attributeName` as `final`; add `Assert.notEmpty()`
* Fix `AggregatorFactoryBean` and `AggregatorSpec` to parse the provided processor
for possible `CorrelationStrategy` and/or `ReleaseStrategy`
* Introduce short-cut methods into Java & Kotlin DSL for an `aggregate()` configuration
* Introduce a `FileHeaders.LINE_COUNT` for header to be populated in the `FileSplitter`.
We need this info in the `FileAggregator` to avoid possible overhead with JSON deserialization
of the `FileSplitter.FileMarker` messages
* Test and document the feature
* Improve `FileSplitter` doc for code block switch (tabs)