* INT-4116: Introduce FileAggregator
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)
* * Rework FileAggregator do not use Java Streams
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