Add configurable custom path functions

[movabo]

Based on #1015, this PR adds the possibility to add custom functions. It implements the separation of contexts as per this suggestion.

Therefore, a PathFunctionProvider replaces the PathFunctionFactory and is now part of the Configuration.

Example:

String json = "...";

Configuration conf = Configuration.defaultConfiguration().pathFunctionProvider(new DefaultPathFunctionProvider() {
    @Override
    public PathFunction newFunction(String name) throws InvalidPathException {
        if (name.equals("toUpperCase")) {
            return (currentPath, parent, model, ctx, parameters) -> ((String) model).toUpperCase();
        }
        
        // Fall back to default functions
        return super.newFunction(name);
    }
});

JsonPath.using(conf).parse(json).read("$.store.book[0].author.toUpperCase()").toString(); // NIGEL REES