Ability to configure indentation

Indent is currently hardcoded:
https://github.com/eclipse-ee4j/jsonp/blob/9892c0658f9d55b819d39039a364df37d1d60202/impl/src/main/java/org/glassfish/json/JsonPrettyGeneratorImpl.java#L31

I prefer two spaces as indendation. To change I would have to replace the entire implementation. It should be trivial to create a configuration property so the indent could be set via the config map supplied to the generator factory.
WDYT?

Start with impl/src/main/java/org/glassfish/json/JsonPrettyGeneratorImpl.java at the hardcoded indentation, then trace how the generator factory consumes its configuration map. Done means the indentation can be selected through that map, including two spaces, without replacing the implementation.

Written by the indexing model from the issue text.