Java DSL: Introduce `FunctionExpression`

Add support for `FunctionExpression` and apply it alongside with `expression`, where it is possible
For example:
```
.enrich(e -> e.requestChannel("enrichChannel")
		.requestPayload(Message::getPayload)
		.shouldClonePayload(false)
		.<Map<String, String>>headerFunction("foo", m -> m.getPayload().get("name")))
```
Remove redundant functional interfaces in favor of `Function`

FunctionExpression: Add JDBC Splitter sample

Minor Polishing to FunctionExpression

FunctionExpression: JavaDocs