Rework shellConversionService

- Replace old ConversionService impl with something more aligned
  with current boot code with partially shameless copy as
  shell is most likely working outside of web or webflux env
  by creating FormattingConversionService and hooking convertes
  from DefaultConversionService.
- This should work on most cases and giving user a hook
  to define their own service.

spring-shell-samples/src/main/java/org/springframework/shell/samples/standard/ConversionExample.java

@@ -41,6 +41,11 @@ class DomainObject {
 	public String getValue() {
 		return value;
 	}
+
+	@Override
+	public String toString() {
+		return "DomainObject [value=" + value + "]";
+	}
 }
 
 @Component