Add toString() method in ArgumentValue

[SnuK87]

Override toString() method in ArgumentValue class based upon the java.util.Optional implementation.

Purpose:
The ArgumentValue class doesn't override toString() which makes logging and debugging quite tedious.

For example, if you define an input type for a graphql mutation with multiple properties wrapped in ArgumentValue:

record MyInput(
  ArgumentValue<String> field1,
  ArgumentValue<String> field2,
  ArgumentValue<String> field3,
  // many more fields...
) {}

You would have to manually write a custom toString() method for MyInput because IDEs or Lombok are (afaik) not able to generate it properly.

[rstoyanchev]

@SnuK87 the change is good, but if you could, please check the DCO requirements, and follow the instructions to sign off the commit

[SnuK87]

@rstoyanchev Thanks for the fast feedback. Updated the Commit as described in the instructions.