Consolidates ConversionService and ConverterRegistry interfaces;
implemented by GenericConversionService.
ConfigurablePropertyResolver#getConversionService now returns this
new type (hence so too does
ConfigurableEnvironment#getConversionService). This allows for
convenient addition / removal of Converter instances from Environment's
existing ConversionService. For example:
ConfigurableApplicationContext ctx = new ...
ConfigurableEnvironment env = ctx.getEnvironment();
env.getConversionService().addConverter(new FooConverter());
Issue: SPR-8389