feat: Add reduceOutputStream option to StringOutputParser

[davidmigloz]

When invoking this parser with Runnable.stream, every item from the input stream will be parsed and emitted by default. If reduceOutputStream is set to true, the parser will reduce the output stream into a single String and emit it as a single item. This is useful when the next Runnable in a chain expects a single String as input.

Visual example:

- reduceOutputStream = false
'A', 'B', 'C' -> 'A', 'B', 'C'
- reduceOutputStream = true
'A', 'B', 'C' -> 'ABC'