JIRA: https://jira.spring.io/browse/INT-4297
* Add `FileSplitter.setFirstLineAsHeader()` and extract the first line from
the context before `iterator`.
* Populate the extracted header to each subsequent line
* Don't use `Streams` to read first line
* Add XSD and Java DSL option for the `firstLineAsHeader`
* Document the feature
* Add Java DSL sample for the `Files.slitter()`
Add a note about more complex header enrichment
Doc Polishing and fix field name.
d8382e0bd0