Add support for configurable indentation length in turtle serialization

### Version

5.0.0

### Feature

Hi I would like to ask for an extension of the configuration regarding serialization of Turtle (possibly including Trig) s. https://jena.apache.org/documentation/io/rdf-output.html#opt-turtle-trig

The configuration option I propose is the number of characters for the indentation which is currently a constant in WriterConst. It is set to 8 and we would like to configure the serialization (of pretty turtle) to have another indentation.

Thanks,
Denis

### Are you interested in contributing a solution yourself?

Perhaps?

Start by reading WriterConst and the RDF output documentation linked in the issue, focusing on how Turtle and possibly TriG serialization options are configured. Trace the pretty-Turtle serialization entry point and identify the existing fixed indentation value; done means the indentation length can be configured and is applied consistently to the requested serialization formats.

Written by the indexing model from the issue text.